You can guess when it is ready to harvest by waiting 8-9 weeks after flowering starts. Or wait until 75% of the pistils turn brown. If you want to be more accurate you have to check out the trichromes with a microscope.

I use a 60x-100x zoom microscope, but I heard that 30x is good enough. Trichromes have 3 stages and they will change color. They start out clear, then turn milky white, then turn amber. If you harvest when they are all clear you will have a less potent bud. If you wait too late, all the trichromes will turn brown and start to fall over. Early harvest is head high, late harvest is usually couch-lock. I would wait until 50% of the trichromes are milky white and 50% are amber.

I would also stop feeding the plant nutrients and flush them two weeks before you plan to harvest. Hope this helps a little bit.