When they start falling off the tree is a good indication they are ripe. Others may have tips as well to know, but thats how I always judge concerning trees.

Wait til you see them fall as Carolyn said. Then sow them immediately. Germination can take as long as 2-3 years, so put them in a sturdy container protected from critters like squirrels. I used a large planter box and covered it with wire rather than plastic. It's sitting in the edge of the yard. I had one seed out of 200+ germinate this year. Hope to have more next spring.

moonphase, try harvesting some from the tree when seeds are ripe but before ready to drop on their own - I'm not sure how to time it in your climate but when the tip of the 'wings' (samaras) are beginning to look a little dry and brown but before the whole thing is completely brown - it's early-ish October here. When the seed has fully dried, the germination inhibitors are fully developed too. Too soon and the seed isn't ripe, but too dry and they take forever to germinate...with a little luck and practice you'll get the hang of it. Sow immediately and they will be given warm moist, cold moist prolonged periods and may very well germinate that first Spring.

HOW do you sow Japanese maple seeds? Break off the thin "wing" part? Soak before sowing? Cover with a sprinkling of soil or sow on top of soil?

I used to get zillions of sugar maples from the seeds that fell from my trees in Massachusetts, they needed no help whatsoever. But I get the feeling Japanese maples are trickier.

There are some nice J. maples at the library here, and I can see the seeds ripening on the tips of the branches, I'd like to try collecting some.

dawiff, you can break off the wings, their only function is to aid in flight :) Wind dispersal. No need to soak if sown fresh, and seeds can be covered lightly, like 1/8" or so. I usually press them into the soil of a prepared pot and top with approx 1/2" of grit, discourages moss and algae forming on pots that are going to be with you a few months.