What caliber and where can you find the serial number on a 1917 amberg 8144 Gew 98?

You are describing what sound like a Mauser 98 rifle. The caliber can only be truly determined by having a gunsmith make a chamber casting. Most were in caiber 8mm Mauser, but there were different calibers, and at least 2 different bullet diameters. Have it checked by a competent smith. Serial number should be stamped on the receiver.