I never found a reliable way unfortunately... I go to supple to the touch but still firm. I think I read something about the tip coming off easily, but not too sure.
Anyway, failing that there is always avocado bread or cake to use them up!
Just type "avocado bread" or "avocado cake" into the EYB search and you will be amazed!
There is even more so of those recipes because of the gluten free, vegan craze. Personally not a fan but I love avocados, and I hate food waste so there :)