Is my iPhone 3GS battery ok?

Hi brought a second hand 3GS and it worked great no problems really battery wise then one day I started noticing the battery worsened I'm not sure if this is just me physcologolicly just being over the top and it's fine. I unplug it at 10 ish and it will be on 10% at around 11 -12 ish I'm on IMessage an social networking through the day along with it being locked for a bit in between is this average battery? I'm a bit causious about what I go on because of the battery so might be why it lasts as long and if I wasn't as cautious it may not. Is this normal performance if not what will benefit it replacement software restore…

Try letting it run flat then giving it a full charge. Also, I think the 3GS does multitasking to double tap the home button and then close all the apps you have open by holding them down and pressing the little x's that appear