One can always cut off a bud, grind it and try it. With practice, one can almost feel when it's close. Then give it the smoke or vape test. Before curing, it might be harsh, but if it burns/vapes well, then it's an indication that it's close.
This is mostly how we do it. TBH, we also have a drying tent with humidity and temperature controllers. We keep the tent as close to 60º F temperature and 60% humidity as we can. At those settings, it takes 7 to 10 days, depending on how much weed is in the tent and how moist it was when it was harvested.