How can you tell if the the bolt is hardened?

I have hardness files, but I’m not sure if the entire bolt should be hard or just certain lug areas

It closes on a Go gauge.
Lacks about 12 degrees to close on a no go gauge.

So headspace seems fine.

Don’t worry about.   The whole soft bolt thing is grossly overblown.  Shoot it.  If you see signs of excess head space you’ll see it soon enough. And you won’t.

I agree with the previous poster about the bolt problems. I am assuming that you are using SAAMI gauges.  Commercial spec, and not NATO to get your results?  That seems a bit tight for a Polytech.  Not a bad thing, but unusual.  Unless someone has already replaced the bolt.  Does the bolt have any letters and numbers stamped into the top?  Or is it bare?  Letters and numbers would indicate that the bolt has been replaced with a USGI bolt. Which usually tightens up the headspace.

NATO gauges

No numbers or markings on the bolt.

HRC50 and lower files skip off the rear of the lugs.

NATO gauges makes sense.  Having the headspace a bit long won't hurt the rifle, though if you are a reloader it won't help brass life.  But, the M14 is hard on brass anyway, so you aren't losing anything.  IF at some point you decide to switch bolts, my recommendation is to try to find a Winchester made bolt.  They tend to be a bit shorter and are a bit easier to fit.  TRW bolts tend to be a bit longer, taking more fitting.
I have 3 Polytechs, so here are some things to watch for.  I would replace the recoil spring with a GI, or aftermarket one from Orion 7.  Chinese springs are a little undersized and soft.  If you have issues with the sight knobs not holding zero, same issue, but Garand knobs work and are easily found.

I agree but you may have the case that a TRW will drop in. I have a poly that wouldn't close on a 7.62nogo (good) but still had a TRW nearly drop in and headspace on a .308

I thinking now decades later the soft bolt theory has been adequately dispelled. I think some did come with snafu headspace because of poor quality control but even then it wasn't the norm. I have an out of the boxy poly that was 1.7-2.0 Moa out of the box and decades later shoots the same. The only reason I change bolts is simply for my reloads and to keep everything the same chambers.

The only thing I detest is getting that set screw out of the receiver and off the barrel threads. Usually a PITA in all that I've messed with.

Too bad I didn't know about this 10-12 years ago, I could have picked one up for $500...ended up passing on it because everyone convinced me it would immediately need a new bolt.

I have owned my Polytech since 1992 and put several thousand rounds through it before having the bolt  changed out.  My primary reason was to tighten up headspace, and improve brass life.

Just a few thoughts here.

As long as your bolt does not close on a 7.62 NATO filed guage, you are safe to use 7.62 NATO ammo.

But stay away from .308 brass.

.308 brass is thinner and it may separate when fired in a longer 7.62 NATO chamber.

A .308 field gauge measures 1.638", whereas a 7.62 NATO field gauge measures 1.6455".

So check your headspace every 500 rounds or so.

If you fit a USGI bolt to a Norinco barrelled action, you will drastically tighten headspace.

My Norinco barrelled action with a Norinco bolt would close on a 7.62 NATO field gauge.

But with a fitted TRW bolt, the same Norinco barrelled action will not close on a .308 no go gauge.

I had one years ago. I would be more concerned about the hooks on the hammer wearing out prematurely. That happened to me.

About 500 rounds into my Polytech that I bought back in 93, went full auto on me.

I ended up getting a TRW trigger group and other than that my Polytech was a great rifle that I wish I would have kept.

I replaced my Norinco hammer with a new old stock Beretta Garand hammer.