Source: Planescape - Adventures in the Multiverse
2nd-Level divination
Casting Time:
1 action
Range:
Self
Components:
V, S, M (a razorvine leaf)
Duration:
Concentration, up to 1 minute
For the duration, you sense the presence of portals, even inactive ones, within 30 feet of yourself.
If you detect a portal in this way, you can use your action to study it. Make a DC 15 ability check using your spellcasting ability. On a successful check, you learn the destination plane of the portal and what portal key it requires, then the spell ends. On a failed check, you learn nothing and can't study that portal again using this spell until you cast it again.
The spell can penetrate most barriers but is blocked by 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.