Check the receiver rollstamp and proof markings. A "real" German WWII mauser K98 will have the waffenamt army acceptance marks (small spread eagle on a swastika with numbers). See that it is complete and unmodified (cutdown stock, missing handguard, drilled receiver, etc).
Late war production items generally have very rough machined finishes, laminated "plywood" stock, and stamped trigger guard/floorplate and barrel bands. Earlier production have better finishes and milled parts. If the earlier production, and all matching numbers, it's a bargain at $125.