You had me at Shiraz.

*makes Katie McGrath a regular, casts Erica Durance*

Also me: 

It’s pretty commonly assumed within the Supergirl fandom that Cat does, in fact, know that Kara is Supergirl. Here’s a list of reasons that makes sense. Keeping in mind that Cat is an exceptionally smart woman who figured Kara out once already, these pieces add up to a very obvious picture.

1x01 - Pilot

Supergirl first appears, saving a plane containing one Alex Danvers (whose name is likely on the flight manifest if Cat were ever to look for it).

Supergirl comes onto the scene immediately after Cat tells Kara that to save the Tribune, she needs a hero.

Supergirl is approximately (well, exactly) the same height and age as Kara with the same coloring.

Kara takes the name “Supergirl” personally enough to actually argue with Cat (as Cat notes in “Hostile Takeover”).

This episode marks the first appearance of Kara’s signature earrings, which Supergirl also wears.

2x01 - The Adventures of Supergirl

Kara is close with Clark Kent, who is publicly connected to Lois and Superman. When Clark comes to town, Superman and Supergirl suddenly team up for the first time. 

Kara says “in some areas of my life I feel so strong and confident”, then lists career and love as areas where she doesn’t know what to do. Coming off of Myriad, the superhero part of her life would be a natural area for her to feel strong and confident. 

“You have integrity to right wrongs and to see justice done. You inspire me, Kara. I can see the hero within you.” Combined with her playful eyeroll at the last sentence, this is very pointed phrasing.

SHE KNOWS.

Below, a few thoughts on when she may have figured it out, and why she’s keeping the secret.