I've have had a couple of poor condition Webleys blued and confess I rather like them. Sacrilege to some but there you are :-)
That said any nice unrestored proper patina gun always has so much more character, interest and desirability.

My re blued guns have some clear signs. Mainly the fact that there is not a mark on them. No scratches on screw heads, the blue is almost black with no signs of uneveness or discolouring where its sat in a box for years, no marks where there is metal to metal contact during cocking.
Also some crispness of engraved text is lost. As you say close inspection of screw heads usually identifies where a worn screw has been filed and sanded. The crisp geometry usually ends up rounded.
Hope helpful.