Identifying objects in sprite sheets

[jeapostrophe]

First, I apologize if this is a stupid question. I've tried to search for the answer on the site, general internet, and the forums.

Now, the question, when I look at most sprite sheets on this site, such as

http://www.spriters-resource.com/nes/sup...sheet/6189

or

http://www.spriters-resource.com/nes/sup...heet/21195

There does not seem to be any "index" into the sprites. For example, I would expect that there would be some formula such as "Each sprite is 32x32 with a 1 pixel margin, starting after a 50 pixel header". In some cases, this can be discovered, for example:

http://www.ladyada.net/images/fuzebox/SMB-Tiles.gif

has 16x16 sprites with 1px margins.

Is there some standard way to identify the individual objects?

In the second Mario one linked above, I could imagine writing a program that would look at the transparent border and the background color on each object to identify the offsets for the pieces, but that strategy doesn't look like it would work on the first one.

Am I missing something obvious or is the answer just "hard work and manually figuring it out"?

Thank you very much,

Jay