Q: Is it possible to tell if something is a capital O, a lower-case o, or a zero (0) without any other letters around it for reference?
A: some fonts have a slash thru the zero, most fonts have the zero taller and thinner, most fonts have the lowercase o perfectly round, the shape of the capital O is usually slightly wider than the zero and sometimes slightly taller than lowercase o, but without any reference it might be impossible to know for sure unless there is a slash