Since I had nothing to lose and I needed more batteries, I figured I would try them. They were physically in good shape, with no corrosion or leakage. I had no time nor inclination to follow standard engineering protocol and measure the open-circuit, no-load voltage. I just popped them into a conventional incandescent-bulb flashlight (not a high-efficiency LED unit).
I was very pleasantly surprised to find the flashlight came on full brightness. Of course, that doesn't tell me how much underlying capacity remained in the batteries (amp hours) but still, to get full brightness from old, albeit unused alkaline cells, there has to be some reasonable amount. It also tells me that their internal resistance was still pretty low, which is both surprising and a good thing, since increase in that parameter is not only a sign of aging, but a major contributor to reduction in terminal output voltage under load.