who and whom

Hi,
It has not been so easy to get a grip on whether I should use the word 'who' or 'whom' in the cases like this.

The person in the center should guess who the leader is.

My guess is that what comes after the word 'guess' is supposed to be a noun or noun equivalent (if there is such one) and from what I see, what comes after 'guess' is a clause, "the leader is 'who'" or something similar to it but not quite a clause, "who the leader is," We seems to use a sentence pattern using the latter part quite often with an example being "She should find out who he is." I still don't know why it has to be 'who' and not 'whom' in the above sentence even after having said what I have said. .