Recognize the subject that the verb is modifying. Take the sentence, "The subject and verb are in agreement." In that sentence, the subjects are: "subject" and "verb," that is, the subject is two items; it is plural.
Scan the sentence to be sure the verb and relevant subject are in agreement. In "The subject and verb are in agreement" example, the verb is "are" to agree with the plural subject.
Modify the sentence if appropriate. If the sentence was "The verb are in agreement," the subject ("verb") and verb ("are") would not be in agreement because the subject is singular and the verb is plural. For that sentence's subject and verb to agree, you would have to change, for example: "The verb is in agreement," or "The verbs are in agreement."
Recognize certain words that stand in for subjects, such as "each." In the sentence, "Each was in its place," you see that "each" was modified as a singular subject. That's a consistent rule not only with "each," but also with "everybody/anybody/nobody," and "everyone/anyone/no one."
Look for subjects that are grouped together by phrases such as, "in addition to," "as well as," etc., maintain the subject's original number. For example "Ben and Steven don't care for skim milk" uses a plural verb while the sentence, "Ben, as well as Steven, doesn't care for skim milk" uses a singular verb.