How to tell that your External Battery Case is charging or full?

I bought a external battery case the one that charges your iphone 4 and i'm not sure how to tell if its charging or dine charging.

With out the external battery cases brand its hard to tell, did the case come with instructions? If you know the brand the products info should be online. If it has a light on it it should change when fully charged red to green, blinking green to solid, or some colour to no colour etc