A Clarki is much more laterally compressed than a sebae. Looking from head on, a clarki = () while a sebae = ( ). A clarki has a lot of forehead above the eye. A sebae, not so much.
Clarki
sebae
If a clarki has a yellow tail there is usually a white bar in front of the tail. A sebae has a yellow tail (or sometimes black) but never has a third bar.
The mid stripe of a sebae slants backwards. The clarki's stripe is vertical.
The tail fin of the sebae is not forked at all. The clarki has a bit of a fork in the tail.
Clarki
Sebae
The biggest thing is the body shape. It is not always so easy in very young fish, but remember that a sebae should have the body shape of a saddleback clown.