If you aren't within 7-14 days i'd be surprised, but you need a 60x magnification to know for sure. Each plant is a bit different, but when the top buds show a little amber, that's enough time for it to cascade down. Perception of what a "little amber" is varies of course :)
google trichomes harvest and you cna read 50 varied opinions of when it is best. Can guesstimate backward from that relative to the ballpark suggestion given above. Do you like a late harvest or an early one can make it difficult to say on top of all that.