insect damage, you can usually tell. the main danger in nuts from the ground is potential fungal activity, and the toxins that can bring (aflatoxin, largely). the problem is that the ground is where all the nuts end up! if you have an 80-foot tall oak, the only nuts you’ll have access to will be on the ground. to me, that means i
should collect what i can and keep my standards high when sorting. after all, some of those nuts landed there just before i picked them up.
i’ll add that acorns can bruise when they fall in a way that makes a dark spot (probably a tannin thing) that doesn’t decrease the edibility if the acorns are processed within a few days, but that’s always just on the one side they landed on. dark blotches in a couple spots on an acorn is no bueno.