i bought a little battery tester thing a couple of years ago pretty cheap under a tenner. This should be able to test the state of the plates inside. Looks like a syringe with a thermomitor inside it, most car spares places cell them i think.
http://www.abbeypowertools.co.uk/batter ... 61792.aspx
And if your really bored this tells you more about batteries then you'd ever want to know :-)
http://www.batterystuff.com/tutorial_battery.html#6
Other wise pop down to either a bike shop or someone like halfords and they should be able to drop test it free of charge.