Identifying key cels

I know this is a silly question, but how do you know if you own a key cel? I never thought about this before, but now I'm curious.

Generally the number is circled on the drawing. This generally (usually?) doesn't carry over to the cel tho. I tend to crop my images, so I can't point to anything in my gallery (altho, for example, the cel of the psy kids in my Akira section is an A1 where the drawing has the circle and the cel doesn't, it just doesn't show up in the scans).

Perhaps someone else can point to an example in their gallery...

An example of non circled cel when the drawing is:

http://gront.rubberslug.com/gallery/inv_info.asp?ItemID=164184

A clearer example [not multiple layers, number is easy to see]:

http://gront.rubberslug.com/gallery/inv_info.asp?ItemID=164815

After checking my collection, I see that I have quite a number of them. Thanks for the info :).

I just wanted to let you know that Key cels can also be marked with a triangle around the cel number on the sketch (example: C4), and sometimes it would even say KEY right next to the number. Another common one I've come across is right after the cel number there is a ' (apostrophe) symbol (example: A12'). These are also examples of key cels. :)