I should next-day air you some ripe ones in a modern case of sending oranges/citrus to people for Christmas.
Anyhow, ripe avocado is soft when lightly squeezed. Squeeze it in a similar way you judge a ripeness of say a peach or other stone fruit. About the firmness of say a ripe banana. I think another good way is to knock it around in a clawed hand. Anyway you do it, it should not be hard like say an apple or hard peach. For guacamole, I say it shouldn't even be firm. I think another way to describe it is to imagine butter. Out from the fridge it is rock hard but once room temp, it cuts and somewhat/is spreads easily. That's more or less what you want out of a ripe avocado.
The avocado skin also should not be brown since it can indicate a bruise. As for an overripe avocado... it should be really obvious. Really soft with browning skin and separation of the flesh from the skin. Now what is not obvious is an a matured avocado and one that is just past it's prime.
I hope all my comparisons helps you form a frame of reference in your mind. It takes time to figure it out. For me, I can glance at some to get a general idea then a touch test to narrow it down. One thing you can probably do is get a paper bag and toss them in with a banana to speed things up.