For tonight, I’m going to rule out instrumentation except for compass, since all the one-design boats, that I’m aware of, sailing at CYC disallow these masthead devices.
First, almost every top sailor is very good at “feeling” his or her boat when racing. They use their senses of touch, balance, sight and sound to know how fast the boat is going, how much pressure is in the sails, how much the boat is healed, how much helm there is, etc. You need to be able to feel subtle changes in speed and angle.
If you can bear off slightly without losing too much speed, it’s probably a good thing to do. It puts you on a more direct course to the mark. Conversely, if if you can head up a little and increase your speed noticeably, you’re probably going in the right direction.