Photoshop :: How To Measure Real Bit Depth?

If I take an image that's 8 bits/channel and convert it to 16 bits/channel, Photoshop will tell me I have a 16 bits/channel image, even though it only contains 8 bits/channel of real information (I suppose the other 8 bits are all zero).

If someone gives me a 16 bits/channel image, is there any way that I check how many bits/channel of real information there are in it?

8 and 16 bits aren't the only options. For instance, I believe my Nikon CoolScan V ED film scanner is documented as providing 14 bits/channel. And, of course, some images may have fewer than 8 bits/channel.