If you press CTRL+ALT+F2 (or some other F?-key from F1 to F6), you'll get a text login prompt which probably reads something like "Redhat 9 (Shriek)" or "Redhat 8 (xxxx)" which identifies it. I recall this is the default.
You can also check out which package versions are installed and compare those to the information on the web; which versions were on RH8 and which on RH9. Or you could use
to get information about the kernel, and if it's a stock one, find out does it belong to the RH8 or RH9 installation. I think the easiest way is to use the text-login prompt information.
Oh, and you'll get back to X by pressing CTRL+ALT+F7.