No universal way to tell if we are talking about having the power cord substitute for an antenna. Best bet is go somewhere with moderate signal strength and try it. Remember the signal bars on most phones aren't very accurate or reliable so you could not see an improvement or see more bars due to unrelated changes.

Most phones have a separate plug for the antenna or they have a way for you to attach the external antenna to the built in one. If you have a mini-usb connector for power there is no antenna connection at this point. That doesn't mean the trick won't work, just that it depends on a lot of other circumstances being just right. If you have a custom connection for your phone there may be an antenna connection.

With out the connection point what is basically happening is the dangling power cord is acting as a reflector or wave guide and providing a bit of a passive boost. This will be very geometry dependent. I would expect straight cords should work better than coiled ones but haven't sat down to work it out. My last real work with RF fields was a long, long time ago.

As I said - I wouldn't bet on it but it is at least worth a try if nothing else is working.

- Eric