Sepa

Sepa, of course, is a present subjunctive form of saber. However, standing alone it also works as an informal way to say you don't know something.

• ¿A qué hora cierra el súper? — Sepa. I have no idea.
• ¿Who is that? — Sepa. Beats me.

Another concise way to say I don't know is Ni idea, no idea. There is also the more dramatic No tengo ni la más mínima idea, I haven't the foggiest idea. When you ask for directions in the street, you may be hear No sabría qué decirte, a distant but polite way of confessing ignorance to something. It's better than being led on a wild goose chase.

¿Quién sabe?

There's a particular gesture I associate with ¿Quién sabe? The eyebrows are raised. Both hands are at chest level, with the palms facing the listener and the elbows flared out a bit. Keep an eye out for it.

Sepo

Saber means not only know but also taste (of). Sabe muy agrio. It tastes bitter. What's interesting is that you aren't limited to third person forms. Sabes a chocolate. You taste like chocolate (when I kiss you). With this meaning, the first person singular present form sepo rather than the expected sé is very common, despite being deprecated by some speakers. Sepo a chocolate. I taste like chocolate. I've also heard sabo for this, as if saber were regular. Perhaps it's best to think of saber as two verbs for these purposes.