I think all sequence-start cels and end cels are key cels - 
the start cels need not be only "A1," which is just the 
first (bottom) layer - a B1, C1 are also start key cels, 
etc. (e.g. my Misaki cel is a B1 key). End cels are *hopefully* 
circled on the cel or have "END" written on it; not always, I 
hear, but usually. 

Middle cels are not necessarily key cels - I think this 
depends on the number of frames in a sequence (if there's only 
four frames, I doubt they would need a middle key). Also, 
I think it would be impossible to tell a middle key without 
the sketch, unless it's circled on the cel, something which 
I've never seen myself...