I have read that Quora accounts have been hacked. How do we know that we are hacked if our password is not changed?

All the things that Jordan mentioned. Sadly it seems that Quora has no way of checking info about logins from suspicious places (they might email you if you suddenly login from somewhere far away but I’m not sure). They should definitely add that.

If you login with Facebook or Google you can check the logins from those places:
Google - Last account activity
Facebook - Account Activity