Anyone that likes avocado probably knows how tough it can be to pick out a ripe one in the store and feel confident that they aren't buying a bruised or over-ripened ball of mush. This little trick might save you some frustration and money.
The window of time for a ripe avocado is short. They go from little baseballs of inedible bitter hardness, to brown mush in a matter of a couple days if you happen to forget about them. This makes ripening your own a bit tough. It's not impossible by any means, but usually I like to pick out a ripe one I can use right away so I don't have to worry about forgetting them in the cupboard or breadbox where you left them to ripen.
"The Nub"
While I am sure it has a proper name, the Nub is the little cap you see at the ?'top' or the smaller end of the avocado. Pictured here at the lower left:

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Once you've found an avocado you think is ripe enough, all you need to do is lift up the nub to expose the flesh underneath. If it looks green like you'd want your ?'cado to be, more than likely the whole thing will be as long as you don't feel any major bruises, or extremely large soft spots. You don't need to squeeze hard to check for bad spots - just a little squeeze to see if it will give slightly under the pressure. If it barely gives at all it's not ripe enough (like a baseball). If there is no resistance whatsoever it's too ripe.
If you lift up that nub and see brown or even streaks of brown there is a very good chance that one has passed its prime. Here is an example of the color you want:
That's it! Just check under the nub.
Using this technique for the past five or so years I have been very successful at picking them out. I've told a few friends and they have all said this helps so I thought I'd share here.
Credit given where credit is due: This was passed to me by my good friend who was told by a "little old lady" at the store while they were searching the ?'cado bin.
Feel free to add in any recipes for guacamole or anything really! No holds barred.