Along with the attitude/demeanour thing mentioned by Rhaedas, once you spend a bit of time around them they each have an obvious 'tell'.

• Morgue has long flowing fur unlike the rest.
• Skeeter is shaped like a seal (and is on a diet trying to fix that).
• Eddie and Gary are both lithe and sleek, but Gary is a solid 1/3rd smaller which becomes apparent when any other kitty is nearby. ANd his eyes are a dark orange while everyone else has yellow/green eyes.
• Batman is quite a bit larger than everyone else and his age is showing in his fur.

Even a bunch of tabbies can be confusing until you know their personality, and then it's easy to tell.