I don't think that it's very difficult to differentiate between the regular massage places and those offering the more erotic massage. I'm a bit surprised that you encountered so many places with male masseurs - most massage in Thailand is done by women. Regular massage places usually offer various types of massages and will have price lists for full Thai massage, shoulder massage, foot massage, etc., and the masseuses usually wear white jackets. Most hotels offer a massage service and some wats and many institutes for the blind have massage facilities as well. You can even get massages on the beach - sometimes the massage is not the best, but it is pleasant to get massaged under the palms while listening to the surf. The places offering erotic gay massage will usually have the phrase "boy massage," gay sauna or spa for men somewhere in their advertising and often there will be a few young men in tight shirts and shorts standing or sitting outside beconning to customers.