How to know for sure that a chicken have a foot infection, without doing surgery?

[Luckybaby]

I performed surgery on two of my chickens had a scab under their foot few months ago. I dig deep, and cut some fibrous wall( one of my chicken had it), but I didn't see any necrotic tissue, or pus. My parent's bought this house about 4 years ago, and I never expect that someone had buried nails and broken glasses,before I put my chickens on the pen that my dad created. I think two more of my chickens have a scab on their foot, which is probably punctured by nail or broken glass. Since 3-4 weeks ago, I found about 25 nails and several broken pieces of glass. I know, that swelling and limping are one of the signs, but I don't think they show those symptoms, though, I am not sure, if my chicken's foot are really not swollen due to infection.