And how do I really know it's time to replace it? It's actually very simple. Take it out and run your finger across the top of it.
If you feel one or two bulges, the battery's toast. It has accumulated so much gas and heat inside from previous abuse the battery is ruined.
In extreme cases, it'll look like this:
A good battery would be completely FLAT. On both sides.
And yes, this is a genuine Motorola battery.
Obviously this trick is not 100% accurate, but it should be an easy way to spot a worn out battery.