There really is no easy way to tell the difference of white bass and hybrids under 15". Usually when they get larger, you can tell their outward appearance a bit more. More pronounced lines on the wiper, and dark back. White perch don't have all of the lines. I think a lateral line only.
There is a tongue patch that is the only true way to tell them apart, but is really small, even in a 25" wiper. I can't imagine it is very easy to distinguish in 12" fish.