Normally you want to get a VERY close look at the trichomes using either a microscope (expensive) or a jewelers loupe (inexpensive)... and you look at their color. If they are clear, they're nowhere near ready... if they're milky/cloudy, you can harvest but you'll only get a head high... if some of them (10-20%) are turning amber, your harvest will give you a head high with an accompanying relaxation.... if more of them are amber than not, you'll be couchlocked.
Now, SOME strains won't have the trichs produce that amber color... so, in conjunction with looking at them you also want to look at the pistils.... if the pistils are 80-90% brown, you can harvest without even considering what the trichs are doing.... You'll also get a good "fade" going on your plant about this time... the leaves will start losing their green, maybe turning lovely autum colors.. all of this will signal it's harvest time...
And one more thing here... some plants will have a strong smell, others won't.... the smell really is no indication of harvest time! I had one grow where the plant had been smelling wonderfully but then the smell changed to grass and I thought all was lost (it wasn't - smell came back after drying/curing).... I've just harvested one whose smell was only very weak throughout the grow and, when I harvested, had no smell at all... still waiting on that one to dry/cure.. But seriously, don't go by the smell... won't work! Too many variables!
Good luck and congrats!