Well Mike, my best guess would be that IC stands for "Insulation Contact", and it is used to designate a recessed lighting can that is rated to have insulation material placed up against it. On the Non-IC cans, you cannot place insulation against or over the can, it must have room for the heat to disapate. That said, the only difference I have seen between the two cans is that the IC ones have a small circuit breaker inside the lid of the can, which can be observed if you disassemble the can from the bottom (room side ) This is a heat sensitive breaker that will interrupt the light if it gets too hot inside the can. I don't know how you can see the difference by looking at them from the top, unless there is a label. When in doubt, keep the insulation well away from the can. The light bulbs will last longer as a bonus.
BobB the EE