Not sure if there is a built in alert - but you could look at the Cluster config and try use something like :
Resource usage exceeds configuration
or
Host resource overcommitted
We trust DRS to manage the moves.
If you are willuing to do this, you can put a monitor on the actual VMs to alert you when : "DRS VM migrated"
Alternatively, you could try an external script monitoring your clusters using PowerCli
One day I will virtualise myself . . .