Worksheets are a great way to test any grammar point. Write down some sentences that use helping verbs, making sure to use as many different kinds as possible. Some sentences will be in perfect tenses, some will use modal auxiliaries and some will be questions. Once you have a good selection, type them up, but this time leave out either the main verb or the auxiliary verb from each sentence. The instructions for the student should be: Fill in the blanks and indicate whether your answer is a main verb or a helping verb.
Conversation is an easy and natural way to test a student's familiarity with verb usage. Pick topics that will naturally require the use of auxiliary verbs. The "future plans" topic will almost certainly require students to use "I will," "I would like," and "I should." The "vacation" topic should result in "I have been," "I would like to go," and other helping verb constructions. Jot down any errors your students make as they converse; after a few minutes of discussion, write any erroneous sentences on the board and ask the students to correct the mistakes.
A good helping verb and main verb activity is to create a short multiple choice quiz. For each question provide a sentence that uses auxiliary verbs. Ask students to identify either the auxiliary verb or the main verb --- alternate from question to question --- and give them four possible answers. Make the sentences progressively more complicated, and the wrong answers more plausible, as the quiz progresses.
Divide your class into pairs and have the students ask each other questions using the auxiliary verb "do." Any topic that lends itself to interviewing, such as asking a partner about family, will work. Students are required to construct questions as follows: "Do you have any siblings?" "Do you like spending time with your family?" "Did you ever behave badly as a child?"