My take on Cat "knowing" Kara's identity is that she doesn't know...
To me it seemed like she said "You're a real super girl" - not "You're Supergirl"...
I don't know the exact dialog but that's what I got out of that scene...
https://en.wikipedia.org/wiki/Brenda_Strong#Early_works
"Elaine's nemesis Sue Ellen Mischke, the "bra-less wonder" and "Oh Henry!" candy bar heiress, on several episodes of NBC sitcom, Seinfeld"