103 is considered the start of a temp for a cat and a call to the vet is inorder.
If their ears are continueously warm/hot to the touch (they can be warm for a moment if they are sleeping soundly etc). Body itself can be extremely warm to the touch when the fever is 104-105. Thumper was with her's at 105.

I got a really nice digital therometer at WalMart that has a flexible tip on the end. Very hand for a wiggly animal. It takes 10 seconds for a reading. Best $15 (tops) I spent.