I recently called a "visiting" escort and it ended up being a girl I had seen advertising at multiple sites in the DC area.  How do I confirm beforehand if the girl is really visiting?

First, thank your lucky stars you weren't set up for something worse. The situation you face is fairly typical. It's called the "agency trap." Anything goes, cause it's all about the money. You have to find the quality 'indy' providers visiting your city; many and often I am sure. Ask this board or around to others that dabble in the hobby. I am reluctant to ever use agencies now because early on I got the belle da jour rather then the one I picked from the menu. Most situations were fine, but a friend of mine told me that many of these agencies recruit street walkers (HIV, Drugs) and clean them up just enough to place them in decent hotels to get top rates. So, you remember the game Russian Roulette? When you don't know what you are getting, your dick gets about the same odds.