If the room isn’t dark and her pupils widen, that can be a clue.Just remember these are just my opinions, but basically think about your own body and think about things you can't really do on purpose. Like if someone tickles your stomach, your abdominal muscles will contract in a certain way. See if you can make them contract on purpose without anyone tickling you. So, when you are with a woman look for those little involuntary muscle movements that can't or is hard to replicate on purpose without certain stimuli. Things like moaning and hip movements are easy to do, but muscle shakes and vibrations are not. Wetness is a response and not necessarily an indication of orgasm or even arousal so that's out. If her pussy is squeezing you, that can be indicative, but you need to add other contextual clues like shakes and stuff.
Tremors in thighs but not nearby muscles.
Oh, & returning the tip money! A sure sign. Lol