Near my work there is an avocado tree growing out over the street that goes unharvested by its owners. I kind of want to try one of the avocados. How do I know when I should go there with the fruit picker and get one down?
Avocados don't ripen on trees, so as @GrahamChiu said, you need to pick it and let it ripen. It may be that the owner has deliberately left them on the tree to extend his eating season, as (apparently), they can survive 3 months on the tree waiting to be picked.
Different avos have different ripening characteristics - some (like Hass) go dark. Most give slightly when pressed when they are ready (but this can cause bruising I think). The best way I've discovered for all avos I've had the privilege of getting my hands on is to see if you can easily pry off the "nub" at the top of the avo where it connected to a tree. If it comes off with only a little pressure its ripe, otherwise leave it a while longer.
Assuming that you have sought permission from the tree's owner, then I'd suggest looking for a large enough avocado with a slightly pebbly skin. Avocados don't ripen on the tree but once they're picked. If it ripens within a week, then you've picked at the right time. It will still go soft if you pick it earlier, but there may be little taste present.