The most reliable way is to visit adult book stores / adult theatres in your area. That will give you first-hand evidence of gloryholes, video booth play, etc. at a given venue. There are some sites that maintain info on GH and related opportunities, Squirt is one, Dr. Emilo Lizardo's blog is another, and there are more sources like Yelp (Google is your friend here).
The reason for in-person recon is to see how safe, clean, and what the activity level, protocols, etc. are at each venue.
For various reasons, the number of GH and other anon sex venues is on the decline.