How can I tell if the battery is flat?
Try turning the lights on. If they only glow dimly, then the battery is in need of nourishment.
The lights may be bright, but do they almost go out when the starter motor kicks in? If so, then it's time to feed the battery. Jump leads or bump start time.
A hydrometer will measure the specific gravity of the battery acid, but who carries a hydrometer with them?