As Speede5 alluded to, half the battle with 3way wiring is knowing how it was done. You should be able to test your assumptions of such with a multimeter; if the other end of the "red" wire was indeed capped, it should yield zero volts regardless of what other line you connect it with on the multimeter.
From there, I'd just use the multimeter to then confirm which is your incoming supply and neutral lines, and make sure they're not dependant on any other switch in the box or elsewhere in the house.
If you're concerned about the power being dependant on a switch elsewhere, just test that assumption if you have access to it (obviously), or simply tag off another supply line that's always hot in the box.
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