Get a jeweler's loupe. They're cheap, at for a pair of 30×-60× loupes. Or, if you have q good camera on your phone, you can use that and take macro pictures. They will help you see the trichomes on calyxes. You want the majority of them to be cloudy/milky. At least a 90%/10% cloudy/clear ratio.
Now, you just need to figure out what kind of buzz you're looking for. If you like couch lock, wait until 25% of the trichomes on calyxes are amber and the rest cloudy.