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I can't seem to find it anywhere?
It's 5 + Caster Level of the Potion.
So, a Fly Potion with a CL 5 would have a Spellcraft or Craft (Alchemy) check of DC 10, for example.
Note that even though potions are limited to spells of 3rd Level or below, the creator could set the Caster Level at any level from the minimum required to cast the spell, up to the creator's Caster Level.
So, a CL 20 Fly Potion would require a Spellcraft or Craft (Alchemy) check of DC 25, and the creator would need a Caster Level of at least 20.
You'd also need the Brew Potion feat, and you'd need to have the spell prepared (or known, for a spontaneous caster).