As long as the four guardians are alive, the door that leads to the inner sanctum won't open. Only the blood of the guardians opens the lock. The crucial point is to locate the guardians. They are hidden at spots where no one would suspect them, disguised as ordinary creatures that inhabit this world. The Eye of Xandalos might be of help to locate and identify them.
If close enough to the guardians, it works like the "find person" spell. Considering the cost of its usage, it is out of question to use it without further thought. The construction of an Eye of Xandalos is an entirely other issue though.