Circuit for turn on a led when load connected
Use a high side current sense amplifier (usually best with an instrumentation amplifier) followed by a comparator and switch.
Here is the basic idea:



simulate this circuit &ndash; Schematic created using CircuitLab
By using a current sense amplifier, the resistor sensing the current can be small (less than an ohm).
Set Vtrip such that when the load current exceeds some threshold, the comparator turns on the transistor.