There is an on board battery looks like ½ AA size. A dead CMOS prevents a boot up. Also a CUDA button can be pressed. Search Apple Support for procedure. Depending what G5 model look at the power supply to see if there was leakage if this a liquid cooled model. If it leaked it may have killed the power supply. Check this site http://www.xlr8yourm...1.html#storytop
This is provided the power supply works. I think DVWarehouse may have power supply exchanges. The pinouts on the power supply are not the same as generic peecee but some have made it work you would have to google that.
If I remember when you remove the plastic case the model and serial number is around the edge of the case. Try everymac to get more info of that model.