Hi.. Without a microscope or jewelers loupe to look at the trichomes, then anything else is a guess. Some people go by the color of the hairs (pistils), or you can lookup the strain your growing and get an approximate harvest time.... If you harvest too early, them you could be losing potency, so you best bet is to get some sort of magnifying device and check the trichomes. Good Luck!
I have a cheap 30x that was like 5 bucks total...anything 30 or above you can see the color fine http://www.ebay.com/itm/New-30x-Ill...353?pt=LH_DefaultDomain_0&hash=item415c7f8d49