Wait 2-3 weeks and take your first reading. Wait another 3 days and take another reading. If readings are the same, it is finished. If 2nd reading is lower than first, it's still fermenting, so give it another week and repeat. You know it's done when you get two straight readings that are the same over a period of 3 days.
Carefully open the lid and dip a sanitized measuring cup to pull a sample. You only need about 1/2 cup or so. Or, get a wine thief and use that to pull your sample. Either way, you'll need to open the lid. If you want to really be cautious, take a spray bottle of Starsan and spray around the lid before opening it.