Put a small resistor in series with the pumps (with small i mean low resistance, 0.1 ohm or lower .. depending on how much current these pumps use; but it must be a powerfull resistor capable of dissipating the power).
When a pump is activated, the current it draws will create a small voltage drop over the resistor, this can be measured and used as a feedback to the control part...
When no voltage drops over the resistor you know there is no load
When too much voltage drops over the resistor you know there is an overload...
Of course; if those pumps are really powerfull (talking industrial over here) the resistors you must use will cost quite a lot, an take up quite some space