This really seems like a stupid question as to the smart alack in the back of the class would answer, “When it does not turn on”.  Sure, this could be one indicator but anyone who is knowledgeable about engines or small engines knows that this is not always the case. To get an indication of if your battery is dead, go ahead and sit on your machine, depress the clutch, and turn the key to start it. If you notice that the engine is cranking slowly and you notice a small clicking noise, it is probably dead. But, it may simply need to be recharged (if it is a rechargeable model). To see if it can still store a charge, hook it up to a battery charger and allow it to rest for up to eight hours. If you come back and it is still dead, then you will need to replace it as it is expired.