The answer to this question is simple. You say so during the conversation. A friend of mine goes so far as to say "no cum, no tip". I had a guy turn up yesterday who evidently was the guy in the profile pic - but the pic was clearly several years old. We met in the lobby of my condo. I sent him away, having already told him that that's what would happen ("Pic this year? No photoshop?"). Similarly all those "massage" guys who send messages offering bargain basement prices of "only" 2000 or 2500 baht. They get a standard response - "so sorry, had sex yesterday with guy for 1000 baht". Most immediately drop their asking price. Interestingly I've started encountering Taiwanese escorts online - the cheapish hotels of Silom Soi 3 is where they hang out, same as the Vietnamese, and similarly extravagantly priced. Generally however I stick to the tried and true boys I've met online and had before