I got serious about foraging acorns for the first time this fall. When ripe acorns started dropping, I started picking. I say ripe because not every acorn that falls is ripe. Early drops, which often have caps still firmly attached, tend to be of poor quality. It's better to avoid these early imposters, and wait for cap-less acorns to fall. (Ripe acorns can sometimes drop with their caps still on, but when you pick them up the caps will typically separate easily, instead of holding tight.)
Once I locate a tree dropping cap-less acorns, preferably large ones, like those of Northern Red Oak (Quercus rubra), I inspect the acorns for signs of spoilage. If the tops are raised, sunken, blackened, or otherwise not good-looking, or if the shells have small round holes, left by exiting weevils (pictured above), I'll leave them be. If I'm unsure of the quality of the nuts, I'll crack a few open to see what lies within.
If last week's quiz had you stumped, perhaps now you can pick out the food grade acorns in the following photos.
As you can see from the following photo, three of the acorns were keepers -- the acorns in the top left, top right, and bottom center. Notice that viable acorns can have slightly cracked tops. Did any of the acorns fool you?
Once I've gathered my haul and returned home, I fill my gathering bucket with water. The water both cleans the acorns and offers another means of identifying discards. Any acorns that float are likely compromised; those that sink are likely keepers, though a portion of them may contain weevil eggs, which may hatch if the acorns aren't processed immediately.* The clean acorns are now ready for in-shell drying, or immediate shelling and further processing into food.
Rather than provide you with all the acorny details, I'll refer you to foragers who have much more experience collecting and processing acorns than I. Samuel Thayer's 41-page acorn chapter in Nature's Garden (2010) is a must read if you want to process acorns with efficiency. If you'd like a video overview of the process, with some useful tips for New England foragers, watch From Tree to Table by Arthur Haines.