^ThisWithout a time graphing machine you are left using some sort of app or the NIST time web page. Set the watch to an electronic clock as suggested then check it in either 12 or 24 hours to see if you are + or-. If you already know the watch is fast or slow it's still good to know by exactly how much. Be sure to have proper tools for opening the watch and the actual regulation. Check your tools with a compass for magnetism so you don't do more harm than good before you use them on the watch. When you move a regulating device, screw, lever etc. don't go for huge movements of same. many watches will react positively to very small changes made to the leaver if they do not then there are other issues at hand such as extreme beat error or bad need of service cleaning oiling etc.
Good luck and work with magnification and plenty of light.