The most obvious way is to test with a second known-good topcase, but I'm assuming you don't have one of those around.
I imagine if you knew what you were doing electrically (which I definitely don't), you could trigger the pins in the plug on the motherboard in the same way that pushing the power button does (like jumpstarting a car, basically). This is possible with iBooks, but the motherboard plug on MacBooks is more complex than the two simple iBook pins, so I would proceed with much caution as far as trying that, since doing it wrong would probably fry the board.
Maybe if you took it to an Apple store while it's apart, they'd have another topcase in the back that they could use to test it? Most Apple stores do repair onsite, so it's possible they would have one.