You need to inspect the heads of the trichomes and make sure they are of correct color and ready for harvest.

Here is a chart to help you better understand trichomes and their stages of ripeness:

Usually you want to choose the strain you grow based on the relief properties you will benefit from most. If you are searching for a high uplifting energetic cerebral high, a sativa dominant hybrid will be your best choice.

However, if you do harvest your cannabis early (when the heads are developed, but still milky white) you will have a much less heavier clearer energetic head high, this may be something you may be intrested in doing, however if you are already growing sativas, I would advise against this.

Hope this helps, feel free to ask if you have any further questions,
Bryan:jointsmile: