In order for some characters, particularly primes, to use the gates that dot the planes, they've got to be able to find them. Hence, an unknown wizard created warp sense, a spell that analyzes the gates and portals of the planes. When cast, the spell allows the recipient to sense any gate or portal, active or not, along a 60-foot path, dead ahead. Scanning in a single direction (out of four in a complete circle) takes one full round, so the person can shift directions while scanning. However, he cannot move in any other way without disrupting the spell; full concentration is required.
Once a portal is detected, the affected person can try to deduce where the other end discharges and the key required to use the portal, if any. Each piece of information requires a separate saving throw versus spell. If one fails, the caster can't learn anything about the other question. The player decides which question is to be asked first. If the player character has actually seen the portal in operation, a +2 bonus is applied to that check.
Note that this spell only functions on gates and portals. It does not detect conduits, vortices, paths, Astral color pools, and other plane-spanning means, and the mage gains no information about them.
Notes: Uncommon for extraplanar spellcasters and dimensionists; otherwise, rare.