if he never changed the barrel you can tell by caliber
The .300 Winchester Magnum chambering also has a non-traditional 280 mm (1 in 11 inch) or 36.7 calibres right-hand twist rate.
For the .338 Lapua Magnum chambering a non-traditional 305 mm (1 in 12 inch) or 36.4 calibres right-hand twist rate was selected to optimise the rifle for firing 16.2 gram (250 grain) .338-calibre very-low-drag bullets. As of 2009 .338 Lapua Magnum barrels can also be supplied with a 254 mm (1 in 10 inch) or 30.3 calibres twist rate optimised to stabilise longer, heavier very-low-drag bullets like the Sierra HPBT MatchKing and Lapua Scenar .338-calibre 19.44 gram (300 grain) bullets. The traditional 254 mm (1 in 10 inch) has since become the standard twist rate for the .338 Lapua Magnum chambering