I'd never eaten an avocado until I was an adult. And after that, it was a really hit or miss experience for me. I didn't like my avocados under ripe and as a result I would generally let them ripen and ripen until the skin was entirely dark and the area around the stem was quite wrinkled and the fruit was softish. Then I'd open my avocado up and occasionally I'd get a decent avocado, but much more often I'd have a nice rotten piece of fruit. I blamed stores for this--stores with their hopelessly bad avocado selection. This went on for years.
Then, the last time I visited my sister, she showed me how she could tell if an avocado was ripe. And it worked. Every time. Every single time.
Your avocado is ripe when the little stemmy area is just about ready to fall out. If you wiggle it, it will start to pull off and sometimes even pull off completely. Then your avocado is ripe and you can cut into it with full confidence. Every time. Every single time.
(See, when I wiggled this guy, the stem came off, but it took a little wiggle. By which I mean, I didn't touch it and it fell off--if that had happened, it would have been a little over ripe.)
Ahh, avocado perfection, right there.
By way of warning, I should say that you must start with a decent avocado--one that is a nice, firm fruit. Not one that is bruised or squishy.
Also, for this to work, you've got to get an avocado with a little stemmy thing still on. I haven't found this to be too much of a challenge.
The great thing about this tip is that it also helps with meal planning. If you need the avocado soon, then pick one from the store with a wiggly-about-to-fall-off stemmy thing. If you need it in a few days or a week, pick one with a stemmy thing that is still pretty firmly stuck to its fruit.