Use a bore light and take a peek down the barrel.
I have an old Springfield 1903 with a worn out throat. The riflling is worn substantially and the best 100yd group is around baseball size.
I don't think the rifle is worth re-barreling so I keep it around for milk jugs.
I shoot a barrel until the groups start opening up, and I can't solve the problem by adjusting my load, or by cleaning the heck out of the barrel.
My last .308 barrel, a Krieger, ran up over 5600 rounds before it was swapped out. Last match I used it, I shot a nearly perfect string of fire at 600 yards, but with almost no X-ring hits. Target looked like a doughnut, with a ring of hits in the 10-ring and only a few hits in the X. The throat area looked like a dry lakebed through the borescope.
A factory Rem barrel (another .308) went south at about 4000 rounds.