You wouldn't be able to totally isolate the flow, because once you did that, there'd be nothing to detect: plugging something in to an 'off' power bar doesn't lead to any power flow that could be detected.
There'd have to be some sort of cycle-on, cycle-off mode of operation - turn the AC on, measure the power, make a decision to stay on or shut off, and repeat.
Measurement is tricky as well - you may well need instrumentation-grade current metering to detect the low current flow, which can also withstand the 15A of a usual circuit without frying.
Some components may draw larger amounts of initial power to get going, before they settle into a quiescent state. Many components hiccup on and off by themselves. The detector will have to deal with all of these variables as part of its logic.
Also, the control circuitry (and AC controls) will themselves consume power. You have to figure if what you can potentially save is worth the power budget needed to save it.
Finally, whatever you have plugged into this power bar will need to be able to withstand numerous perpetual turn-on and turn-off cycles. This is quite stressful for power supplies in general (due to inrush current).