Hi Erica, welcome to SAAFE.
Have you been in London long?
The best way to find busy agencies is to look up Punternet and Punterlink (use Google) and search those sites for their field reports, which are themselves easily searchable so you can see the most recent reports for escorts from agencies (excluding all the indies) in whichever city you are interested in. You will then have a list in front of you of the agencies that are popular with punters and, by checking the reports' contents, a vague idea of what the girls working for them are like (i.e. if all the reports complain of the girl being surly or rude, you might be looking at an agency that takes a high cut and pressures the girls to work more, or other negative possibilities such as bait'n'switch/scam operations). You obviously have to read between the lines for most of this, unless the punter is very unhappy in which case they do spell it out.
For a more direct approach, you'll probably have to go ahead and contact an agency you're interested in and do a bit of sly grilling during the interview - as you've said you have plenty of experience with the industry, you'll probably be able to see through the usual blab the agency owner tells new girls, about how awesome their agency is and how many millions of pounds you'll make etc etc. Or even just keeping an eye on the agency's website for a while can show the number of new girls they recruit and how many 'old' girls get removed from the site (this is often due to punter complaints about rip-offs/fakes), plus how many of the girls' pictures look fake (Tineye can help - Google again!) or how generic the profiles are can perhaps show a lack of interest in promoting and advertising the (honest) girls who have been on the books a while.
I'd be interested to know if US agencies are different - here in London, most of them make their money by continuously hiring newer, younger girls to interest their clients and because of this, it's not unusual for girls to frequently reinvent themselves and join a new agency, to keep their New Girl status. These are amongst the many reasons that I went indie after doing agency work - as an indie, it pays to build up your business and promote yourself with honesty, which is refreshing.
Hope something here can be of help to you, and good luck with your next steps.
Let us know if you have any more questions!