Make sure you are following the law. Hiring an escort is only legal when you're paying for her time. The whole industry is a time-based industry where you pay per hour, day, or week for your companion's time.
Is she legal? Make sure that she has the proper identification that states the validity of her stay here. If she is from a different country, that is completely fine, but she has to have some sort of identification.
If you go through an escort agency, your chances of finding a professional go up dramatically. There are a lot of 'self-hired' escorts, that are very new to the industry, and that can cause you to lose satisfaction if you were expecting someone that has experience.
Number 4: Make sure that you're able to get a background check before hand. Usually, the agency that is hiring the companion will ask for a background check before you can make any interaction with the escort, this is for every party's safety.