This is as much a factor of how Fey work in your world (i.e., I'm a big proponent of VRG: Shadowfey, which is totally different) and how accessible you want them to be to outsiders. At the very least, you want to give the Fae a saving throw against being discovered, and possibly a series of saves based on the level of knowledge aquired per stage of the spell, with different Fey having a better/lesser chance of avoiding detection.
Fey don't work the way other base types do, especially in my campaign. Trying to track a Sidhe would be near impossible, as Fey tend to come and go as they please, and have a series of complex advantages over mortals. Weaker (not necessarily smaller, not necessarily bigger) Fae may be easier to track because they won't be able to resist the spell as easily. Remember that finding the aura, and determining the strength of that aura, are two different things.
LCpt. Thia Halmades