How to determine the "native" size of a bitmapped font?

I have a couple of bitmapped fonts in the form of *.dfont files. I would like to find out the "native" size(s) for these fonts. By "native" I mean the sizes (in points) of the bitmaps themselves, without any scaling.

I thought that Font Book's ⓘ tab would provide this information, but it doesn't (or it is wrong: the only size information it gives is, invariably, "18pt", which I can see is obviously wrong).

Is there some other way?