To start with throw some new spark plugs in just for the heck of it. NGK BPR6ES if you're close to stock.

Pull the plug wires one by one and see which one isn't firing properly. Pull that one at the coil pack and see if it sparks to the wire when you hold it close to the coil. If it does, replace the spark plug wires and see if that fixes it.

If you're not getting spark from the coil, you may have a bad coil pack or a bad power transistor, which is close to the coil pack and has a long flat connector in it.