one way is to see when the growth slows. I dont know how close you have been able to watch your plant, but during flowering they go thru very rapid, consistent growth, about 1-3 weeks after that slows down is a good time. the plant is pretty much done flowering and you wont miss out on any potential buds.
another way is to look at the resin glands, or trichomes. When they turn a milky white to an amber color, its time to harvest.
Some people say when 75% of them turn amber, thats a good time. But id have to say its more personal preference.
Heres one thing to keep in mind. Too early or too late you wont get the peak amount of THC. When they are a milky color, you will get more of a cerebral, soaring kind of high.
When they are more amber, its like a couch stone. you are just kind of stuck, bodyish high.
go off what your personal preference is, or read up on it on this site.