Checking the battery with a voltmeter will tell you the static voltage but with no load on the battery, it's a meaningless number. That's why, when you think your bike battery is going south, you engage the starter with the multimeter hooked up to the battery. That way you can see what the voltage falls to under load.

 

My local Wal-Mart will test a button cell battery for free (jewelry/watch department). The tester puts a brief load on the battery, and displays the remaining life as a percentage.