Isolate the subordinate clause. Then read its subject, verb, and any objects, restructuring the clause if necessary. Some writers find it helpful to substitute he for who and him for whom.

Samuels hoped to become the business partner of (whoever/whomever) found the treasure.

test:…whoever found the treasure. […he found the treasure.]

Ada always seemed to be bestowing a favor on (whoever/whomever) she worked for.

test:…she worked for whomever. […she worked for him.]

In questions (25b)

Read the subject, verb, and any objects, rearranging the sentence structure if necessary.

(Who/Whom) conferred with Roosevelt and Stalin at Yalta in 1945?

test: Who conferred…?

(Who/Whom) did the committee nominate?

test: The committee did nominate whom?