Let's say – hypothetically – in the vastness of space, we found a planet made entirely out of antimatter and surrounded by barely any matter. If someone were to look at the planet, would they confuse it for a normal planet, or would it look any different?
Larry Niven dealt with exactly this question in his short story Flatlander and as far as I know his details are quite reasonable.
While there is nothing intrinsically different about an antimatter planet from an optical point of view, its interactions with local matter such as solar wind or (for deep space) deep-space gas and dust would leave it rather different from an ordinary planet. Space is not, after all, completely empty.