The most common sign of a bad ignition coil is an engine misfire.
The misfire could be on one or more cylinders or if you have one ignition coil for the entire engine and also have a distributor and distributor cap and rotor then the one ignition coil failing could cause engine misfire in all or one of the cylinders.
Also if you have one ignition coil and it goes bad your engine may run rough, may start but then stall and then start up again.
Or the engine may not start at all.
When the ignition coil is not working properly it's not sending the proper amount of voltage to the engines spark plugs which is needed to start the engine and keep it running.
You can have Autozone or other auto parts stores run a check engine light test on your vehicle if you check engine light is coming on and you're getting hard starts or engine misfire and it can tell if you have a bad ignition coil or not.
Engine misfires, rough idle, and loss of power are very common signs and symptoms of a failing ignition coil or coils.
One of the most common symptoms associated with a faulty ignition coils is engine performance issues.