I've found the floating method to be highly ineffective. A significant part of acorns that float on the water surface is sound and vice versa - part of acorns that go the bottom of the water container proved to be infested (I've just made a small trial and found a 20% error on both sides). In some cases it is probably not a problem. However, if one is limited in his study by the number of acorns (like in non-mast years) and importing some seeds from another part of the country is not a solution - then loosing 20% of sound acorns is too much.
For the moment, I am just examining acorns looking for insect (Curculio mainly, I believe) holes, larvae emergence holes or other signs of being spoiled. However, this method is rather time-consuming and probably still some part of acorns that I consider sound is infested by something. I am working on oaks.
You could try X-raying your seeds. Seeds banks used to do this to check on the % of filled seeds during cleaning. It should work for checking infestation.
If X-raying is not available, you can try absorption method. Instead of standard floating, try to re-dry acorns (a few minutes to several hours) and repeat flotation again. During drying period the empty and damaged seeds will lose water more rapidly than healthy seeds. As a result, healthy acorns will sing very rapidly during second flotation, while immature or damaged seeds float.
You can find an interesting paper regarding assessment of floating method on acorns at: http://www.rngr.net/publications/tpn/46-4/46_4_143_147.PDF/at_download/file.
Healthy acorns will (obviously) sink, not sing during second flotation;)
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