parasitic drain?

My battery has always been at about 12.7 volts, but after i took my car to a shop the other day to see why my car cranks longer than usuall. My battery sometimes reads 12.2 or 12.4. its lower than usual so im trying to find out whats happening exactly. What are some of the possible causes for this? I don't have an ammeter capable of doing a parasitic drain test so if i just check the open circuit voltage every hour for 5 or 6 times would that be enough to see if the battery is draining while it sits there?