There's a stem of sorts at one end. Prod at it from the side - if it falls off easily, it's ripe or too far gone. If it stays attached, not ripe yet. More usef

There's a stem of sorts at one end. Prod at it from the side - if it falls off easily, it's ripe or too far gone. If it stays attached, not ripe yet. More usefully, you can buy pre-mashed avocado in the same style as pre-made guacamole. If the use case involves (or could be made to involve) mashed avocado this option has a serious shelf life (presumably it's irradiated or similar) and lives in the fridge until use time.