key

This is probably really stupid....
How can you tell if a cell or sketch is a key?
Is it marked somehow, or is related to the video?

You can tell by either the sequence number on the cel or the matching douga, which is circled if it's key. Also, A1 and end cels are always key.

So the letter and number is circled on the key cel or just the number?
Thanks for the help ^^

It depends. Sometimes just the number of the cel is circled. Other times, there's a triangle around the number. And other times, both the letter and the number will circled.

In cases where the item doesn't include a sketch, sometimes you can go through the video. You'll need a high quality video to do it, but, key cels generally are better drawn than their counterparts. You'll find that there are very little errors, if any, on your specific sequence. For example, the eyes will be properly aligned, the movement will be smooth, etc.

Also, the first and last cel of a sequence is always key. For example, B1, C1, F1, and anything that has the word "End" or the letter "E" after the sequence number. So, B3End, etc.