In my opinion there are only a handful of things that will help you as there are dodgy looking places that are legit and very legit looking looking places that are dodgy.
Biggest giveaway for me is if the venue gives a health rebate. If it does then it is licensed. Gift cards are another 99% certainty. The only other thing is the location of the shop. If it's located in a mall or hotel then it will be 99% legit. I say 99% legit as there can be rogue girls anywhere that will go the extra mile without the shop owners knowing, no matter how high end the shop, for you to find one is unlikely but still possible.
The only other giveaway is if they cover you with a towel during the massage. If they do this then your chances will be lower but certainly still possible. If they ask you to leave your underwear on then for sure 99% legit.
As for things like tissues, signage etc...it means nothing. One last thing I have found is that thai massage has a higher chance of being legit and Chinese is anything goes.
Also, even if you visit a shop that is popular with the lads, the chances are you will have to ask for it. Only some girls will ask you, the rest leave it upto you to ask.