“Non-verbal cues” have been mentioned above. There are plenty of Tui Na Qi Gong spots that give a good solid massage. I’d say anytime you pass one on the street, immediately google the phone number and see what comes up. Usually there are Google reviews, Yelp, sometimes reviews from this site, and advertisements. If any of the ads are from adult entertainment sites, the place is not what you are looking for. If there is a review from this site, obviously the private details will tell you all that you need to know. If you do not have access to those, if the tip amount or general price range is higher than what a legit spot offers, the place is not what you are looking for.
Now, if you find a place that has only Google and Yelp reviews, the reviews are generally good, and there are only women who work there, I would say go check it out. From there, be relaxed and genuinely friendly, even a touch flirty - not in the sense that you are trying to impress them, but more just at ease and happy to be there with them. When they leave you alone to get ready, remove all clothing including underwear and lay face down without the towel covering you. Be pleasant and smile when they come back in. Keep up a friendly easygoing rapport. You may find that after a solid therapeutic massage on your back, glutes (pay attention to how they rub here and the upper thighs - the very skilled can hit erogenous zones in a very therapeutic manner), and legs in which various knots and kinks are worked out, on the flip they will continue with your front and get subtly a little more gentle and then be very comfortably handsy in certain unexpected yet eagerly anticipated areas. From there, see where that rapport takes you. Do not push. The worst that will happen is that you get a legit inexpensive massage. The best, is that what the other thing you were seeking is freely and naturally offered, and you’ll have your well-rounded therapeutic experience.
You may also find when looking up the number that not only was the spot reviewed here, it is exactly what you are looking for. If so, book the appointment.