Step 1:
Watch the little white aggro bar next to your name
Step 2:
If the bar gets to high, switch to Avatar and let the other 4 dps kill the dread

Seriously though, if the tank doesn't get a perfect snap pull where they can immediately put RoH combo on the dread then aggro can be an issue, especially on the dread that spawns before brain jack where the tank only has about 4-5 GCDs of aggro abilities to drop on. I tend to watch the aggro bars as tank while brain jacked and holler if a dps gets dangerously high.