It's a feel thing for me.
If I feel that I am running a "high cadence" then I am not working hard enough, if I feel that I am "lugging" it, then I need to jack it up a tooth or two.
My fastest hill climb times always felt just right, I was working hard but it just felt perfect. Kinda obvious sounding, but it is true. Downside.......(or upside) is that you have to be in your best shape to know the difference.

Sorry for the vague answer, but there is a golden cadence for everything, and it depends on you.