When the trichomes start turning mostly amber, THC begins to degrade, and the plant begins to die, eventually the flowers will fall off...

If you can't get a microscope then you'll just have to guess. If you bought the seeds from a seed bank, they usually provide an estimate for finishing times, but this also depends on your growing conditions. For example, growing with CFL's will require a couple more weeks of flowering time compared to HID lights.