I found this nifty little trick here and gave it a try yesterday at the grocery store. I have never bought an overly ripe avocado, and now I never will! To tell if an avocado is perfectly ripe, simply find the little "stem" part.
Remove the stem part. If the part underneath appears yellowish/greenish, it is ripe.
Perfectly ripe avocado
If it appears brownish, it is too ripe. Don't buy it!