Typically want to look for a Chinese massage place in your city or close. There are also Korean and Latina spots in some places. I've never seen black or white in non legitimate massage spots. Although that doesn't mean you couldn't get extras, but would probably take tons of sessions, and the massage therapist getting more comfortable with you.(probably pretty fulfilling though)
You'll typically know if there's any chance of getting some extras soon or even that day.
Signs might include: walking into the establishment and seeing a door that separates the lobby from the individual massage rooms
Having the front woman asking how old you are, if you look younger.
Asking you if you've been there before, or if it's your first time.(probably just say you've been there before, but it's been a while)
Having the option to get a "table shower"
Sometimes, but not always starting off with a mediocre or poor massage.
Sometimes, but not always being "teased" during the massage (meaning some lingering touches on the balls, butt, or dick)
Of course all of that could still happen, and when they ask if you needed anything else after finishing the front of the massage, they say no to "extras"
You could also just look at rubmaps and check whatever area you want. With the free version you can just see which places have a lot of reviews from other people who have been there. That's a good sign they have extras. Depending on what you want, your looking at anywhere from an extra $40-150ish on top of the massage fee
And always pay cash