The X-Serve 1U rack mounts when the PSU is working still have fans running, while the rest of the machine is in a off or very low power state. If there is no fan activity you can assume 1 of 2 things
Dead logic board
I work as an Apple tech so i come across these bad boys from time to time.
If you need to get parts, I would recommend finding out the part number (I have dirrect access to apples Global Service Exchange so i can look up parts for you and whatnot) if you want to attempt to repair the machine.
Once you get the machine running, you can run the AXD (Apple Xserve Dianostics) on the machine which is basically a boot environment that then checks a good chunk of everything.
EDIT: Do you know what year the X-Serve is and is it Intel or a Power PC?