You're not likely to notice throat erosion with your naked eye even with good light. You're also not likely to find a commercial .308 that has been fired enough to erode the bore unless it's been used for an extreme amount of target shooting. In this case, there should be other factors showing such heavy use.
I have 3 rifles built using barrells taken off of .308(7.62x51)British military sniper/target rifles. God only knows how many rounds went down the bores and about all that did was make them smoother.