Firstly all of benzene's bonds are the same length and therefore there cannot be both double bonds and single bonds in the molecule. Secondly the bond angles are all the same as well which would not be the case if Kekule's structure was correct. Thirdly the electrons are delocalised around the carbon ring, there are 6 delocalised electrons, one from each carbon in the ring. The structure of the benzene ring is more accurately represented by a hexagon with a circle in the centre as we mainly do now.