nestly :
how to know a power on password?
when you create one for yourself, make sure you write it down and store it in a safe but easy to reach place. create a password recovery disk while you're at it. a password recovery disk will help you in case you lose your password.
if it's a password for a networking device, check it's manual for default password. or ask your network administrator.
if you're asking how to know someone else's password, i cannot help you with that. it is against tomshardware terms to help with password protection bypassing.