usually folks harvest after the tricomes are half or more amber (rather than clear) so start flushing before then.
The way people tell when the tricomes have turned amber they use a magnifier (often purchased from radioshak for a reasonable price) If you have good vision like me, just get the biggest magnifying glass you can find, and look real close, and make sure you have plenty of white light (some bulbs are yellowish and can confuse you).
peace