you will need a magnifying glass to look at the trichomes.. if they're mostly milky with some amber, then it's time..
milky trichomes give you more of a head-high, while amber will give you more of a body high (you shouldn't wait too long until most of them turn amber though).
i prefer about 5 to 10% of the trichomes being amber, the rest milky
from this picture unfortunately it's not possible to see the trichomes in close-up