Two field tested techniques:
1. Hand on the thigh, double-pump squeeze. Usually while sitting after a joke.
2. Egg crack - Make your hand into a fist (like an egg) and lightly knock it on her knee 3 times, cracking the egg, slowly opening your hand and running your fingers across her thigh until your hand is completely open and your palm is on her leg, slightly above her knee. You can use this to lead into the hand on the thigh technique for added effect. You can use this as a joke in itself or as a "ticklish test" e.g. a bullsh!t reason to lay hands on her. Also a heat check. Its not so much a specific reaction your looking for but rather the absence of rejection. She's unlikely to slut out and be like "oh, that makes my pvssy so hot." What she WILL do is NOT move your hand, NOT back away, NOT give u a negative reaction.
This is usually a sign that you're greenlighted to go further e.g. keep rubbing her thigh, keep your hand on the thigh, and make your move/go for the kiss.
This is simply one of many things that's field tested. Happy huntin