Hi,
You can create a workflow rule the Resource link creation. Then you need to create a conditional action that will check if the new task got the resource to be overloaded - this is the code to check whether the resource is overloaded: not (IsResourceAvailableForGivenPeriod($Resource,$WorkItem.StartDate,$WorkItem.DueDate,100, "PlannedWork" ))
In the indented actions you can send an email or have a pop-up notification to the current user.
Please let me know if it makes sense.
Regards,
Tamir