I haven't heard a lot of stories about bad headspace in Mosins (then again, I'm still quite new to milsurps so maybe I haven't read enough yet). I'd fire a round or two and inspect the brass closely, I think if headspace was really bad you'd see signs on your empty brass.
Or, drop by a gunsmith's shop; no need for you to buy gauges if you're only going to check one gun!
Lou