1. Find out who wrote questions. This should be readily available at the home page or by clicking an obvious link. Licensing exam companies pay 300-500 dollars for each question on the test you will be taking. Good quiz prep companies will also be willing to pay a qualified author good money to write their questions. Bad companies will get someone to do them on the cheap, and they may not be up to date, representative, or even accurate.
2. Find out who the company is. This also should be readily availble and there should be a good explanation of who the people you are paying are. Are they a one-off company that decided to make a few bucks doing quiz preps, or are they an established company that was doing this before the internet made this type of thing so quick and easy? Stick with an established company.
One personal note, no matter which companies and books you go with, pay less attention to their USPAP info. With as many changes as have been taking place, it is much better to read the current version yourself and maybe make your own study guide for areas you need a little assistance in. You will more than likely have a couple of USPAP questions that will be regarding recently changed items to make sure you are up to date. The bonus is, if you get into an argument with someone, and they tell you to read USPAP, you can say that you just did.