The general way people tell is by looking at the 
douga.  If the sequence letter/number are 
circled, the cel is considered 'key'.  An end cel 
is generally marked on the douga and often on the 
cel with a little 'E' or 'End' or something 
similar near the sequence number.  Sometimes you 
can't tell if something is a key cel based on 
what you have.

Keys