Prior to fermentation, use a refractometer. It uses just a few drops. I got mine a couple of months ago off of eBay for about $26 delivered from Hong Kong. It makes it very easy to adjust the length of your boil on the fly in order to meet the OG you are shooting for. Once fermentation has started you should use the hydrometer. Any LHBS store will sell you a cylinder for testing gravity. Personally, I don't even think about checking gravity until my beer has been fermenting for about three weeks and I am getting ready to keg or bottle.