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

Having to pay for the swap complicates things.
Is there a trans or at least a flywheel and bellhousing on the extra engine? I've fired them up on the ground before.
I see 2 options, I've done both before at different times.
Swap out engines being you have another sitting there and rebuild the one currently in the car, swap back in when you get the one now in car rebuilt:
Or, rebuild the one on the ground and then swap.
Did you hear any air escaping when you did the leak down? Out the mouth of the carb? Out the exhaust?
Were you sure each was at TDC with the valves closed when you put the air to each hole?