This is a reprint from one of my old articles, I hope ya'll find it useful...

Selecting the Right Personal Protection Dog
Ok, youve followed the advice from the thread Selecting a Trainer/Vendor and after doing your extensive homework, youve gotten in contact with a good vendor. You have been in close communication with this vendor via e-mail and phone and he thinks that hes got a dog that you can work with.
What now?
There are a few steps that I suggest you take to make the most out of your visit to see the dog.

Come a little bit early to the site and ask to be shown around *before* you see the dog. Look at the kennel set up - it everything clean, is it a place that youd be fine with boarding a dog at?
Take a look at the training equipment, is it well cared for and organized?
Look at the training site, do they have mock ups of building/windows, cars,etc. where they can train realistic protection scenarios? Is the area fenced for safety?

Take the time to look at the dogs paperwork. Check its registration ( if any, there are plenty of excellent un-papered dogs ) and look closely through its health records. Take a look at its hip x-ray rating. If the dog was owned by someone else previously, find out *why* the dog was sold, and be demanding in a clear answer for this ( these are all points that should have been discussed over the phone before you made your trip there, by the way - youre reviewing the paperwork to double-check and verify facts now ).

Let the trainer introduce the dog to you. Observe the dog, is it confident and sure in its surrounding? Does it look you over but also look to the handler to see if they accept your presence? Is the dog the correct weight, is its coat in good shape? Does the dog look well-exercised and not like its been in a crate for 23 hours a day?

Once the dog has become familiar with you, its time for your decoy to do their job.
Yes..*your* decoy - to do this the right way, youll need to bring an experienced decoy that you can trust and that the dog has never seen.
Now look, this dog probably has received some meaningful training, but the odds are that its been all on a single decoy ( not many vendors have multiple skilled decoys on their payroll, a good decoy is hard to come by ). What we want to see is how the dog reacts from a threat by a decoy that it doesnt know.
So youll stay far out of the way and the handler will have the dog on a strong leash. You call your decoy via cell-phone to come in and the decoy will start to loudly knock on the door to the office - watch the dog, hed better bark here. The decoy escalates the door pounding and the prospective PPD had better be letting your decoy know to go elsewhere.
Finally the decoy forcefully swings the door open and talks in a loud and threatening manner - prospective PPD should be displaying outright aggression and dragging the handler towards the decoy ( all of these reactions by the dog are without commands ) who then leaves.
The handler now calms the dog to the point where you can pet it, etc. The decoy has changed into his bite suit out of sight and is now outside around the corner of the building or behind a van ( whatever you and the decoy and the vendor have decided on previously ). You and the handler and the dog walk outside and stroll calmly to the spot where the decoy is hiding - the decoy jumps out and attacks the handler and you watch for the reaction of the dog ( a slight startle is ok, but the dog has to recover immediately and bite the decoy. Ideally this happens so fast that the dog doesnt even zero in on the fact that theres a bite suit present, so speed is of the essence here ).
I highly suggest that you, the handler/vendor and the decoy walk through the exact steps of the scenario at least twice so everyone knows their part.

A dog that passes this test is the dog that you write the big check for.