Buy a jewlers loupe or get a decent magnifying glass. The most accurate way to tell when to cut is by looking at the trichomes heads. A lot of people will give you different answers as to what color you're looking for and at what percentage of color you want. The truth is that it depends on a few things.
First let's discuss what colors mean what.
Clear heads indicate that your plant needs to flower a bit longer as the cannabinoids haven't reached their delta form (9-delta-thc etc).
Cloudy heads will have more thc than the clear heads and amber. This will produce a very cerebral, active high (Strain phenos still play a roll in the affects).
Amber heads mean that the thc has started to oxidize into cbn. This will produce the typical stoney, narcotic, couch-lock high that every is after.
Now, you should decide what sort of high you're looking for in regards to when you should harvest. If you want it more uplifting, harvest when the vast majority are cloudy, with a small percentage of amber (To ensure most of the thca in the plant will mature before you cut). If you're like me and you need to go to sleep or want a medicine that's very "stoney" then try to harvest when half or so of the heads are amber. Though it should be noted that Sativas and hybrids with heavy Sativa phenos will more than likely never fully amber. Some strains will die before they ever amber. So if you're a sativa grower, beware.
This chart should help for a visual reference
