In some places, it's very obvious, it can often be read on the signs outside. If a place is offering "testicular massage" chances are it's the seedier choice. If it's unclear about what they offer, other than simply telling you "massage", and the girls are scantily clad, it's also likely a seedier massage place. Then there are the places in between, they're kind of seedy looking, but it seems like they offer regular services. Your first sign at that point is how well you can see into the establishment. If you see the massage chairs and other people getting massages inside, you're probably OK. These middle ground places do however sometimes offer both legitimate and seedy services. In that case enjoy your massage, and maybe the experience as well.
If you're worried about accidentally getting involved with a seedy place you can always play it safe. There are several spas in Thailand that give you the full serene experience, candles, calm music, all that relaxing stuff. These places look very fancy and are often more expensive than the local street massage place. A good place to be really sure, if you're worried, is to go to a reputable hotel. Hotels and resorts in Thailand often offer spa (including massage) treatments. You'll pay a lot more for them, but it's likely that there will be nothing seedy about these places.
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