well, again, it's a gay sauna, so it is okay so make eye contact, to stare (in a sensual way, not a creepy way..), when you see someone you like and he likes you back, you will know. As you both wear only a towel, and once you get hard or he get hard, or start play with yourselves, you will know
As for the 'no' part.. it is okay to say no, to someone who approaches you and you're not interested. So far, I never come across a guy who I told him 'no' or 'sorry' and try to force himself on me. There is an etiquette between guys in such places - consent is always the no. 1 rule.