How to judge the Trainers Expertise?  

The average dog owner is not aware but, yes Dog Trainers do compare each others skills and abilities, through a variety of international dog sports. 
A Dog trainer that wins first place at a local dog show does not say much.
Winning a national or a world championship is a whole different story. Competing against the best 120 dog trainers in the world and winning is a statement for sure! While such win doesn't bring any money, it brings the recognition and envy among other trainers. The problem is - for the last 20 years there are only 20 world champions.
As client who is willing to invest serious money in a dog, You deserve accurate representation of the trainer’s expertise. Schutzhund, Ring sports, KNPV and few others are competitive protection dog sports that exist to determine who the top trainers are. 
Some of the sports are recognized internationally and some are pretty much local mediocre events .
Sometimes trainers will use the term World Champion in their credentials ( as I do). Simply ask the question :
"Is it individual placement or a team title?"
A team title, means that at least two of the team members were good enough to put the team in 1st place.
But it is the individual placement that matters most. As an example I have won 2 individual world champion titles, and the team from Germany was the 1st place team. The point is I was better as an individual than all of them .....

Being a sport trainer is not enough! Training the dog for real scenarios means the trainer has to have that knowledge too. So what else should you look for ?
Involvement with Police, SWAT teams, ATF tactical , Secret service, the army... Just as in the Dog competitions - don't be fooled by someone who was holding the leash  of an already trained dog during patrol. Look for the instructors  who teaches them all.
It is really the combination of the two fields protection dog competitions and Police K9 work that you should look for.

Contrary to what you might have read - Owning a protection dog does have some similarities with owning a gun. Although the dog should do the job - you still need to learn some dog training skills but even more important  there is a whole part of tactical training that needs to be learned and it is something that requires knowledge as a dog trainer and as tactical intructor
As an example view my Credentials and click on the diplomas to enlarge.

Now I want you to go back to your research. You might notice that most companies will try to sell you $40,000 to $65,000 executive level dog, when the trainer has no idea of what this really means because they have NEVER really trained a dog to high level of competitions and Police work themselves...

I am sure you have already seen "Level 1" , "Level 3", executive protection, and so on all over the internet.
There is no unified system for rating the level of the protection dogs! One company’s Level 2 dog might be very weak compared to another company’s Level 1 , and "executive" is guaranteed to cost more but not really give you information of the level of training of the dog.

You can judge a company’s  standards by checking out the knowledge of the trainers, which will first be evident in their accomplishments.

Sport dog versus the real dog

One thorny issue is that of the sport dog versus the “real” dog. Contrary to the way you should judge the skills of a trainer (competition accomplishments), dogs that have done well in certain sports may actually not be suitable as protection dogs.

In a nutshell, many dogs that do well in dog sports may be only “sport” dogs, meaning they will not protect you or your family if needed. They will not bite an intruder or attacker without a sleeve or a bite suit.

You may see a very fancy video of a dog that works great on a bite suit, there are few protection sports in which the dogs bite a full body suit. Please DO NOT be fooled - they are still sport dogs! Beelieve me, there is a huge difference between sport and personal protection training.
This is where a skilled and honest trainer is necessary to tell you about the dog in question. Unfortunately most people who are trying to sell a protection dog

As an example, on occasion police K9 dogs with the highest performance records in trials will be purchased by a police force and find themselves, say, chasing a suspect in a dark alley. But when the dog finally catches up with the criminal, it becomes highly confused and stressed, because it wants to bite him, but it doesn’t see the familiar equipment. These cases are NOT rare, they are the proof of how important is to select trustful  source for your protection dog.

What about videos and pictures of the dog for sale?

Pretty much any dog of sound temperament can be used to some extent as a protection dog, or at least as an alert dog. The smaller breeds (up to 50 lb) obviously would not be “man stoppers.” Any larger-size dog could be a protection dog. Most dogs must go through vigorous training in order to understand when they are needed and how to be of help.

As an example, on occasion police K9 dogs with the highest performance records in trials will be purchased by a police force and find themselves, say, chasing a suspect in a dark alley. But when the dog finally catches up with the criminal, it becomes highly confused and stressed, because it wants to bite him, but it doesn’t see the familiar equipment. These cases are rare but they are evidence of how important is to select the right training source for your protection dog.

The dog has to have natural protective instincts, be pack-oriented, and/or territorial. Even an Afghan Hound could do it (a rare case, but I have seen it done). Generally speaking, and only generally speaking, some breeds make better guard dogs, (meaning they would do well protecting their territory), and others would make better owner-protection dogs (meaning they would do well protecting their owners).

Two of the universal protection dog breeds are the German Shepherd and the Belgian Malinois. At present, these are the two breeds that stand out from the rest. Around the world it is not an accident that 99% percent of the police and army dogs are mostly Malinois or German Shepherds.

Yes, a Doberman Pinscher or a Bouvier, among others, could do just as fine a job, but the temperament of these breeds has been compromised in recent years by breeding for their looks, and the prevalence of genetic illnesses such as hip dysplasia has risen dramatically.

A dog from a mastiff breed can be very strong and a great deterrent, but their trainability level does not come close to that of a Malinois or a German Shepherd. Therefore, they are limited in what they can be used for in terms of protection.

Another plus for the Malinois and the German Shepherd is that they have double coats, meaning they can function in any climate: snow or desert. They have been bred and used for protection from the late 1800s, and through the decades and around the world they have been selected for their temperament, trainability, agility, strong bite and scent work, and their lack of fear.

It is a fact that in recent years, the Malinois has become the number one choice for protection work due to their work ethic, trainability, and natural protectiveness.

In any case, you might find a dog of almost any breed that can serve as a protection dog. But please take the following qualities into consideration:

• Stable temperament.  The dog should not be performing out of fear, and it should be easy to handle and live with
  
• Balanced Energy Level. Your dog should not be bouncing off the wall at all times and at same time he should not be sleeping if someone is ringing the door bell.
• Good size.  A dog has to be big enough to be effective.
• Speed.  As with an athlete, the weight and speed of a dog have advantages and disadvantages. A dog that has a bit of both is the most universal type, therefore the Malinois  are clearly the breed of choice.
• Health.  The Malinois have fewer health problems than any other working breed.