How do you know if your 1943 copper penny is real?

Take it to a reputable coin dealer and he can verify it for you. Some easy tricks to do is to see if it sticks to a magnet, if it does, it is a fake. Secondly, look at the coin under a loupe and compare the last 2 digits to a genuine 1943 steel penny, if they aren't identical, it is an altered coin.