How would you detect vampire/standby power?
I think you're laboring under a false assumption. Where there are power strips that turn outlets off to save power based on a 'control' outlet, the 'control' outlet is always live.  It will always be consuming its 'phantom power'/'vampire power' (cue the Twilight fangirls!).  All of the outlets that are slaved to the 'control' outlet will be off until the 'control' outlet has enough current flowing through it to trip the circuit.  As others have suggested, this is the only reasonable way to do it - you can't be continually switching something on and off just to test whether it's on.  

I have such a power strip for my home entertainment system - older solutions were to run power through your receiver via a built-in pass through.  When the receiver is off, the passthrough is off, when on, it's on.  I can't say for certain whether it's saving me any money but it definitely means fewer absurd 'off' lights on the electronics (seriously, a light that's on when the device is off?  Crazy).

If you want to measure this 'phantom'/'vampire' power, get a Kill-a-watt.  It measures power draw of whatever is plugged into it.  I haven't used one yet but many people rave about them, and Lady Ada hacked one to put an Xbee unit in it for datalogging purposes, so they're at least popular.