Symptoms of a bad coil pack?

Your engine will run poorly, rough, lack power, etc. as if it isn't running on all cylinders (because it isn't). It may or may not start poorly, or at all. It will likely be burning more fuel than normal. One or more spark plugs may be blackened with dry, sooty deposits (indicating poor ignition). If spark tested, one or more spark plug leads may show weak or non-existent spark.

Engine diagnostic tools will often be able to point to a coil pack as being problematic, but often can't tell conclusively. They will often simply say that some cylinder or another is misfiring.

Note: Before assuming a bad coil pack, check your spark plugs and wires and replace them if they are either bad, quite old, or if their trustworthyness is questionable. Plugs and wires should be replaced in complete sets whenever possible.