To answer your question, "Yes". It is possibly to tell 'if" a weapon has been fired even once. Forensically! More practically in that context all the other "evidentiary" matters as how recently, particular ammo, ambient temperature/humidity at time of firing; and more.
Practically speaking, the term "unfired" is best taken as a good faith 'opinion'. Technically better put perhaps in lawyer language as a good faith perception that the rifle remains in new condition, the firing of which beyond proofing, has not "apparently" occurred.
Continuing "practicality" bent, more literally a "so what". Particularly if buyer intention is to fire it. That unless, "deflowering" an object some kind of achievement!
Where such status has greatest meaning, the more vintage "collectible" guns, typically themselves aged. Likelihood of "unfired" there potentially more meaningful, but requiring an unusual sort of provenance or some such accounting credibly!
"Unfired" the term is of a category commonly called "puffing". That is, of unverified and likely stretching sales technique. Such of a sort commonly encountered in a particular business/trade and not a "representation upon which a potential buyer is necessarily entitled to rely as more than simple "opinion".
"New in box" has more meaning as more easily appearance-verifiable to reasonable satisfaction.
We could venture into all sorts of "what if..." examples for discussion. But I won't. I do suggest simply taking such representations with a grain of salt!
And closing with the aside... What rifle bores are commonly blued???
Best & Stay Safe!
John
