Take a peak at your trichomes with high zoom on a good camera. Or, use a jeweler's lens or magnifying glass. What you're looking for is a milky trichome head(the sphere on top of the spikey structure). If ALL of them are milky and not clear, you're good to harvest.
If half of them are amber and the other half cloudy, you're good to harvest as well. Amber trichomes are a sign that they are over ripe and you don't want your trichomes to age any more than that. They will start degrading and falling apart.
If you don't see any heads on the trichomes, they have already deteriorated and you let your plant flower for too long.
I hope this helps. Good luck!