The best advice you can get when starting on a collecting venture is to buy the best reference books you can get, and study them.
Frequent forums where people who seem to know their stuff can be found and listen to what they say, with your salt cellar kept handy of course!
The caveat is that older books have often become dated due to later research, and sometimes they perpetuate errors. Some of the "coffee table" books on this subject are brimming over with errors and plagiarisms.
Making fake sniper's rifles is a minor industry, and the German Nazi-era kit is the favourite field of these fellows. I would bet there are four fakes for every real K98 "sniper" out there.
If you want get really keen, you start saving photos off the Net, logging numbers etc. etc.