Honestly, I can really tell Kara from Jazz or Jazz from Gigi blindfold.
First of all, they have different voices. Kara chirps and squeaks, while Jazz bleats and Gigi meows.
Then they have different sizes and weights: Kara is very light, Gigi is very heavy and Jazz is just in the middle.
They purr differently, and their fur feel different under my fingers.
Yet sometimes I can make mistakes..