Let's be clear, Thai massage joints are the easiest places to get "extras" to your massage. Most - if not all - of the girls and women working in them have worked in Thailand (duh) so they know the score.
I've been in places that have had signs up saying "no sexual services offered" and I've always had a happy ending.
Look for the signs, and make sure you're giving off the right signs; keks off, legs parted when you're lying face down. Whenever she gets near your junk or your ass crack, squirm and make the right noises. If she's up for earning a wee tip she'll make it clear that extras are on offer.
Failing that, just fucking ask. As noted above, they've worked in massage places in Thailand, they KNOW why men are there. The only place I was turned down was in a parlour in London, near Marylebone Station. I wasn't getting any signals from the girl, so I just asked as soon as I turned over onto my back. She said no, it wasn't allowed there, but directed me to a Chinese massage shop two doors down. (Where I went after the Thai massage and got a fantastic naked body to body with a hot Chinese girl.)