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The FFIL file is what is traditionally know as a Mac font suitcase. In OS 9 and before it generally had a suitcase icon. In the case of a Postscript Type 1 font, there needs to be a LWFN paired with it; this is the Postscript Type 1 Outline.
Another common situation is that the font is a traditional Mac TrueType font. It has the outlines contained in the same file as the Suitcase. There is no obvious way to tell using only the Finder. It does not need to have a .dfont or .ttf extension, and does indeed store critical data in the resource fork. If you add the font to a font manager, you should be able to determine quickly that it is TT.
If a PostScript font suitcase is missing it’s outline, it may sometimes be refered to as ‘bitmap only’ or an ‘orphan’. In some applications, these faces may still be available, typically you will get a warning if you try to print them. This situation exists for backwards compatability with certain applications.