I would like to test a spare battery to see if it's fully charged, can I do it with a regular digital multimeter? If so, do I check voltage? and what is the fully charged value? If it's about half charged what would the voltage be? Thanks.
+1 for having it checked at autozone, but yes, set the multimeter to DC, if you need to pick a range pick the setting right above 10V. With the car off it should be 12V, running it should be higher, 13-14V.
But yeah, pull it and have autozone check it for you. Best way to determine if it's ok or not.
An internet search will turn up several sites with the info. From http://en.wikipedia.org/wiki/Automotive_battery, 100% charge is approx. 12.60 volts, 75% is 12.35 volts, and 50% is 12.10 volts. As the article says, temperature can affect it, but those numbers are supposed to be good for 20°C (68¯F). They give a temp. correction formula of -0.022V/°C.
You'll get an artifically high reading right after charging due to "surface charge" (charge unevenly distributed giving falsely high reading). For accurate reading, let it sit for 1/2 hour after charge is stopped before taling the reading.