There are a couple of ways to do this. If you have a distributor, I have found the best way to find the failing cylinder is to remove a plug wire from the cap one at a time with the engine running and listen to the engine. If you do not here the engine change, then you have found a misfiring cylinder.
If you have a DIS (Distributorless Ignition System) then all you need is a Scan Tool. Plug it in and read the codes for Ford, or use data stream to see the misfire count of each cylinder.