Determine Distro without logging in.
 

I have a set of boxes that are all versions of Centos, Debian, Red Hat and maybe a couple other distros and have been tasked with determining and documenting which box is which OS. However, I cannot login to any of the boxes for privacy reasons or turn them off. Every box has no GUI (console only) so the most I can access is the login screen.

I know it may be impossible, but is there any telltale signs to look for in order to determine what distro is running from just a console login?

Any help would be appreciated.

well sometimes the boot manager says it
sometimes at the login prompt when you boot up it says it

Use a live CD, that's the only way I know. You may get lucky of course and it may say somewhere during the boot process.

you could try to use nmap
it will try do do a OS detection.