Why don’t you just try to change the password? If the old one works, you will successfully change to the new password. If the old one doesn’t work, then you will have confirmed that you don’t have the valid password to the account (bad news!).
You should be able to alter the password with:
ALTER LOGIN sa WITH PASSWORD = ‘<new password>’ OLD_PASSWORD = ‘<old password>’
If the old password doesn’t work, then you can try to do this without the old password and force it to a new password value, as:
ALTER LOGIN sa WITH PASSWORD = ‘<new password>’
If neither of these work, then you have a problem…