You can tell if an avocado is ripe by gently squeezing it - if it gives slightly under pressure, it is ripe. Additionally, the skin color should be dark and the stem should come off easily when ripe.
An easy way to tell if your avocado is ripe is by flicking the
small brown stem off the top. If it comes off easily and the bit
underneath is green, it's ripe. If it doesn't or is brown, it's not
ripe.