Cels have a number in the upper right-hand corner 
(usual position) with a letter followed by a 
number.

Key cel ID:

1) If the number is 1 (regardless of what the 
letter is; letters just refer to what layer the 
cel is in a multi-layer setup.)

2) If the number is circled on the cel and/or on 
the sketch.  In some cases, the cel number on the 
cel is not circled but circled on the sketch.

3) If the number is followed by the word 'END'.