I would still float test them, but most should be OK as long as they don't appear to have any other issues. Also don't force the caps off - you can actually damage the acorn. Most of them should turn color and then the cap will separate much easier. If this was the case earlier in the fall I would be more concerned, but now they should have had time to mature and still be viable. Sometimes oaks will abort their acorn crops if stressed for some reason - those acorns tend to not be viable. Sometimes acorns fall while still green - typically from a wind storm or other critters trying to eat them. If your real concerned and don't want to waste your time. Store them in a cool place and take a few and soak them real good and see if you can get them to germinate - they should germinate in just a week or two with the right moisture and temps - sort of a "rag doll test" like we do for other seed.