What's the best way to find a dead cylinder?

A firing plug just means the plug is firing, not weather the cylinder is dead or dying.
As other above have said, a compression check is the easiest best test. Pull all the plugs, wire the carb open, make sure the battery is all the way up, then have at it.
The numbers you get (write em down) will tell the story.
Edit: a very weak cylinder or two, time to pull the valve cover and check rocker movement, and lash, (if a solid cam).