Is there a simple way to detect which connector is powered?
It's possible to reliably detect microvolt voltages. Say you pick 100uV as something that's easy to do reliably. A 0.5" trace that is 50 mils wide will drop 100uV at 20mA. At 800mA it will drop around 5mV, pretty much negligible. 

Take an op-amp such as the Microchip MCP6V61T- it has a guaranteed Vos of less than 5uV and draws only 80uA. The chip can only take 5V so you'd need a supply, or there are more expensive parts (eg. OPA192) that will work directly from 24V. 

Your circuit could look like this: 







simulate this circuit &ndash; Schematic created using CircuitLab

You may not need the 1K resistors and capacitor but if there is a strong surge (say to charge a capacitor) then you could conceivably exceed the 300mV or so input range of the op-amp. That would be more than 40A with the suggested 0.005 ohm trace resistance, and more like double that with the op-amp ground placed in the center of the trace.