In areas with lots of mages it is usually the 'named' or important ones but yes, killing them all will remove any ambiguity as what I said above is not 100% true. If you explore and gain the detect artefact spell through the Tower of Sorcery you can, after rank 47 find all of the rooms/mobs specifically without a need for the 'pot luck' element of killing everything.

Some classes are not so easily spammed up to rank 47 to look so examining and killing mobs from 30 onwards can/will help you find your sleeks AND limited wands also.

Straklaw, in a post above, mentioned them not being on guildguards or guildmasters (unless I read it wrong) but that is untrue. I know of three or so sleeks on those types of mobs. Never on a shopkeeper yet unless you count all mobs you can buy stuff from as shopkeepers, then even one of those I know has an aura.

It is a pain, we know, but talking to people IC and offering your magic to help others find theirs can help you build up a list of possible places for future characters. That said, my current has all but exhausted my black list and completely exhausted my sienna and I am still without.

Good luck