Your best bet is likely a preemptive strike.
Does the player next to you have an early death mana node hooked up (or worse, death/chaos/entropy)?
Did he go for writing? (Helps with picking up Necromancy early.)
Does he have the Summoner trait? (And to a lesser extent, the Arcane trait?)
Do you see skeletons, or adepts with the death I promotion?
If so, attack before conjurers appear!