Misfire at cruise, popping/afterfire at 3000RPM

It's very likely they're reversed? Is there a way to test and/or correct the polarity without cutting them? The only way I've been told to troubleshoot this is to watch the timing strobe, see if it bounces around. It does a little bit, but I'm not certain it's bouncing around enough to be solid evidence that the wires are reversed.

I supposed I could cut them, switch them, and then see how the engine performs afterwards. And if it's worse I can just switch them back.