Well...
You can only determine "safety" if something is so grossly evident that you can see it with the naked eye. Not likely, if it was purchased from a major firearms retailer.
Headspace, OTOH, cannot be checked visually. You will hear that Mosins rarely, if ever have headspace issues. That is likely true for re-arsenaled rifles. If it's unknown history, you have no way of knowing if someone has changed the bolt or bolthead at some point without verifying headspace.
And yes, I bought a Mosin that failed a no-go gauge...so I check them all.
I would buy, or borrow, a no-go gauge- or for worst case, safe-to-fire, a "field gauge" which allows even more slop.
It's not entirely scientific, but if your buddy won't cop for the $30 for the gauge, tape two pieces of cellophane package tape (usually about 2.5-3 mils thick) to the back of the rim of a piece of BRASS. Then chamber it, and slowly work the bolt closed.
But first- make sure the bolt isn't sticking excessively from cosmoline left in the action, chamber, or bolt. The Mosin doesn't have a particularly impressive bolt throw to begin with, and you don't want excessive resistance.
SLOWLY work the bolt closed. You should feel a significant increase in resistance about halfway down as the lugs begin to seat and press the rim against the chamber. If you don't feel this extra resistance, I would for sure, get a real no-go gauge.
