This simply isn't true. As an owner of multiple legit Thai massage shops, I can confirm that there are plenty of shops that only offer legit massage services. My shops are in shopping centres, with nothing to indicate any extras being available, but they still get asked regularly.
If you want to know which shops offer extras, just do some research on the forums & online. And don't hassle the legit shops.
Your best chance of identifying potential shops which offer extras could be - is the shop open late at night? - does the shop offer health fund rebates? - are the shops staff hold Australian massage qualifications? - Can you see inside the shop from outside, or is it blocked out with photos & open signs? - Does the shop advertise online with focus on the staff's looks? - Does the shop advertise massage services on sites such as Locanto under personal services?