If it's hooked to the battery charger and the charger is turned on it's charging
Are you asking how to tell if it's actually making the plug glow? If in doubt,check it out! Pull the plug from the engine and test it. You can buy an igniter with a meter on it that will let you know if the plug is shot.
And you can upgrade your charger. Get one with a screen. Check out "Venom Pro Charger." This way, there is no guessing. If it's a defective battery, you'll know it.