It's Arcana, but there really should have been an entry for it on its own.
You can choose whatever you want for it, I suppose, but it should be Int-based. Psions need to be good at it, as good as Wizards are at Arcana. And Arcana is the only Int knowledge skill that's even close.
If I were to homebrew a knowledge skill in Psionics, what classes would automatically be trained in it, and what classes should have the option to be trained in it?
I run a Darksun game in 4e. I ran one in 3e, and even back in 2e. One of the things 3e did with Psionics was introduce Psicraft. It was basically the Psionic counterpart tp Arcana. It basically that took the Arcana DCs and uses, and made it strictly Psionic. So, if you needed say DC 20 Arcana to identify a Magical effect, then a DC 20 Psicraft would identify a Psionic effect.
The thing was, You could either have Psicraft or Arcana. Which led to the questions of whether or not a Psion could identify a magic effect, or vise versa. I mean, even a Fighter with a little Arcana had a chance. So there were two options brought up.
1) Interchange them. Consider Psicraft = Arcana = Psicraft. You could use either skill to the uses present for both Arcana and Psionics.
2) Prioritize. Choose your primary (Arcana for mages, Psicraft for Psions) and then when you want to interact with the other power source, you took either half of your skill rank, or your skill rank -10, and then added your Int. The difference in the number used could determine level scaling and difficulties.
Now, in my Darksun game, I let Arcana function in both the Arcane and Psionic fields equally.. mainly for simplicity.