Look for the pistol hairs changing to a brown-ish colour. The more brown pistol hairs you have the heavier the high will be. This is down to personal taste as to how long you leave it as this determines how strong you want your weed to be. Some people go for 60-70% brown pistol coverage before harvesting. Also, get a strong magnifying glass (or a jeweller's loop) and look at the trichomes. If they are a milky white or cloudy then they are most likely ready. How long have you been vegging/flowering?