The selection of who or whom, whoever or whomever is dependent upon the case that is needed in the sentence or clause in which the relative pronoun is used.

• The nominative case is used whenever the pronoun who is the subject of the sentence or clause, or the predicate nominative, a pronoun that follows a linking verb such as to be. Here are some examples:

Miss Skiffins is the person who is in charge of casting for the play. [subject]

Whoever studies ancient history knows about the polytheism of the Greeks and Romans. [subject]

Miss Skiffins is who I am. [predicate nominative]

• The objective case whom or whomever is used when the pronoun is a direct object, indirect object, or object of a preposition.

One of the wise men about whom we wrote was Solon. [object of the preposition about]

You may give whomever you choose this treat. [indirect object (to) is understood before whomever]

Such people as the polytheists had several or even many gods whom they worshipped. [direct object - they worshipped whom]

**Hint:  In order to determine which case to use, find the verb that is the predicate of the clause in which the relative pronoun acts and determine what is the subject.  If there is another noun or pronoun in the clause, many times it is the subject (especially with active verbs), so you need objective case for the relative pronoun.

For instance, in your example Guess (who/whom) I met this morning, say the verb met and ask "who met?"  The answer is the pronoun I, so the position of subject goes to I. Therefore, use whom.  ( I met whom = the direct object)

Likewise, in the next example, whom is used for the same reason:   As an orator he is a man (who, whom) no one can accuse of using a few words where many will do.  No one is the subject of the clause, so the only position open is direct object. (no one can accuse whom = direct object)

In addition, the first two sentences use the objective case whomever you can trust [direct object] and whom you were thinking of [object of preposition of ]  for the same reasons given above.

However, the last example has no other noun or pronoun that can act as the subject so who is the correct choice:

Library privileges will be withheld from those who have been found guilty of mutilating borrowed books. (____have been found guilty -- a subject is needed)

So often we are confused about these relative pronouns because people do not always say the appropriate one and we learn through listening the ungrammatical use.  So, since converstional English is more lenient in its rules, we must go through the little "test" of finding the subject of the verb in the relative clause in order to choose the appropriate relative pronoun.