I do this, I know my Milgauss is +6s/d, on the edge of COSC regulations, which is -4s/d to +6s/d. The process is simple, set your computer to a time server (eg ntp.internode.on.net is what I use, defaults for the major two OSes are time.apple.com or ntp.windows.com) and set your watch to it. 24 hours later, set your computer to the time server again and compare your watch. However fast or slow it is, that's your reading.
For example, if I set my computer to an internet time server at 2:59PM today, and then stop the movement on my watch and set the time to 3:00:00PM, when my computer hits 3:00:00PM I pop the crown and start my Milgauss' movement at 3:00:00PM exactly.
Tomorrow, I set my computer to the time server at 2:59PM again, but leave the watch going from yesterday's time set. When the computer hits 3:00:00PM, my watch will be 3:00:06PM. It is consistently fast, always 6 seconds every day, which is how I know it is +6s/d and thus just within COSC regulations, which is what you want from a replica. I also know that since it is consistent in that, when I eventually get it serviced and regulated I should be able to get it to within 1s/d, which will be awesome.
By the way, computers, believe it or not, are not very accurate. You would think they are based on quartz watches but they can sometimes lose or gain time more than an automatic watch. That's why all modern operating systems come with the option to automatically set the time to internet time servers which are based on atomic clocks, which have unprecedented accuracy. (EDIT: And by the way, I work in IT, I didn't think keeping time was an issue for computers, I only know this because when I set my watch every day it would be 6 seconds or so, then one day it was like 2 minutes off and I'm like "wtf is wrong with my watch?" and I found out that because I had only set the time server manually once, the day I got the watch, the watch was being set to the computer every day which was quickly getting inaccurate, after a week the computer synchronised itself and corrected itself by like 2 minutes, so all of a sudden my watch was 2 minutes and 6 seconds off in one day not 6 seconds off! Now I changed the registry setting so the computer syncs itself every day)
Most people could not give a shit if their watch is anything less than a minute or a couple minutes slow/fast and only set them occasionally, roughly, but I like to keep mine deadly accurate so I take the 15 seconds to set my watch every day. Speaking of which *looks at computer time* she's 3 seconds slow!
