Is it possible to find out if a Mac was simply powered on (power button) but not logged into any account yet?
You can view this information using the last command in Terminal.app.

The last command brings up the login history in LIFO order. Each entry contains the name of the user who logged in, the local time the login occurred, and the duration of the login session.

Shutdowns and restarts are logged as pseudo-users named 'shutdown' and 'reboot,' making it a simple matter to see who (if anyone) has logged in since the last startup, or if multiple shutdowns/restarts occurred without anyone logging in between them.