You will not be able to find out the password he chose. However, you may create a new user or set a new password to the existing user.
Usually, you can login as the postgres user:
Open a Terminal and do sudo su postgres.
Now, after entering your admin password, you are able to launch psql and do
CREATE USER yourname WITH SUPERUSER PASSWORD 'yourpassword';
This creates a new admin user. If you want to list the existing users, you could also do
\du
to list all users and then
ALTER USER yourusername WITH PASSWORD 'yournewpass';