Asking “is this okay?” has never ruined a good moment.

Checking has never spoiled anything that was going well. The worry is that asking will break the moment, when in practice it does the opposite, since it removes the guessing and lets both people relax. A yes that was asked for is worth more than one you assumed. Anything that could not survive the question was not fine to begin with.

Make it ordinary rather than solemn, in any situation where you are not sure: is this okay, do you want to, should we stop. Say it in your normal voice. And when somebody asks you, answer honestly rather than politely.