Go online to Radioshack.com and spend $12 on a 'self-lighted hand-held microscope"
By the time you get it - they May be ready... or see if you have a Local Radioshack and they'll get ya one..
CAREFULLY remove a sugar leaf from a bud about 1/2 way down the plant (the top almost always is ready 1st-- so we use the sides as a guide...) use the scope to look at the crystal formed on the top side of the leaf (Trichomes/trichs) If they look like spheres or lollipops---and are cloudy, they are ready... some people like to wait until a few start to turn amber... it makes it more narcotic...
But you need to flush the plants accordingly-- if 8 weeks- do you mean from day 1 of 12/12 or from when they started to flower?
The "day one" is when the plants start to actively flower-- each plant is different... so even if all have been in 12/12 8 week- some may be 6 weeks along , others 7... either way... if those are in soil.....time to flush!!! They need at least 2 weeks after being flushed of getting just water & carbs (if you use them) otherwise, the plant retains a lot of magnesium, Nitrogen etc.. that effects the taste, aroma and harshness...
Don't jump the gun on harvesting--early/unripe weed is harsher, less potent and takes-on a grassy/hay smell that ripe/well cured weed doesn't have.
They look real good! Enjoy!!!!