How do i know if these shells are .223 or 5.56?

i was digging around in the reloading room and found a coffee can clear full of what looks like .223 but idk if they are 5.56. theres a tag on the top but its been covered in something and i cant read it. all i can read is its a 55 grain fmj bullet. the bottom of the case doesnt have .223 or 5.56 printed on it im sure my dad knows but just in case he doesnt how do i tell?