When you start a gauge swatch, record what hook you're using by tying a series of knots in the tail end of your yarn below the beginning slip-knot.

A "C" hook would be 3 knots; an "F" hook would be 6 knots, and so on. Or if you are consistently metric, 3.0mm would be 3 knots, and so on. As long as you remember what you system is!

It's fiddly to tie the knots but, on the rare occasions that I have picked up an experimental swatch six months later where I had actually remembered to tie the knots, I didn't have to start all over again to figure out how I got that gauge.