Which rote spell (from the Mage: The Awakening line) would be the most effective and "cost effective" (easiest to learn and cast etc) to reveal an Azlu Crawler (from Werewolf: The Forsaken core and the Predators sourcebook) that has already entrenched itself in the skull of its host body?
The Death 1 rote Forensic Gaze. (Mage: The Awakening, p.134) Page 121 of Predators tells us that the process of usurpation by the Azlu inevitably kills the host. Forensic Gaze allows the mage who casts it to know "what killed the being in question and how much time has passed since it died." Since it explicitly works on vampires, it's no stretch to see how it would work in this case as well. ("Cause of death: brain eaten by tiny spider. Time of death: three days ago. And yet you're still walking around.")
Failing that, the Prime 1 Rote Supernal Vision could pick up the "threads of Essence" in the bodily fluids of the possessed being.
Depending on your GM, the Exorcist's Eye spell (Spirit 1) can detect a possessing entity, although your GM may rule that the Azlu's situation doesn't count as possession (despite being described in Predators under the heading "Possession").
The Healer's Trance spell (Life 1) would surely be able to diagnose the medical condition of having your brain actively eaten by a spider.
Note that since you don't need a rote to cast a spell, any of these 1-dot detection abilities are as cheap as the first dot of their Arcana.