Short answer is gauge and thump with my finger. I work with each set of tires to find what pressure "feels" the best for that set of tires on the roads I ride. I then make a mental note of that pressure and pump to that pressure for each ride.

Thump a tire? More like a strong finger flick. Yeah, it's a habit I got in to years ago. After a while I could tell within a few PSI what a tire was at just by the sound that thump made. I still do it each time.