Flip She will come to a question: Will she come? No do needed, and come can drop out: Yes, she will. Both moves only work for auxiliaries, so will is a modal auxiliary verb.
She will come. / Will she come? / Yes, she will.
Spotting the other members
Can drops the third-person -s: She can swim, never cans. Its clue is that missing -s.
May opens polite requests by inversion alone: May I come in? No do needed.
Must takes no to: You must go, not must to go. That missing to is its giveaway.
Might flips without do: Might she join us? It also takes the bare infinitive: She might join us.
Should pairs with the base form and carries no -s: He should rest. The bare rest, never to rest or rests.
Can drops the third-person -s: She can swim, never cans. Its clue is that missing -s.
May opens polite requests by inversion alone: May I come in? No do needed.
Must takes no to: You must go, not must to go. That missing to is its giveaway.
Might flips without do: Might she join us? It also takes the bare infinitive: She might join us.
Should pairs with the base form and carries no -s: He should rest. The absent -s is the diagnostic clue again, now with a different word order after it: the bare rest, never to rest or rests.
When will is not the modal
Now flip a sentence like She willed the door open. The question is Did she will the door open?, so do appears, exactly what main verbs need. Since the auxiliary test fails, this will is a main (lexical) verb, meaning to make something happen by determination.
Another clue confirms the modal side: will is defective like the other modals. It takes no -s, no to-infinitive, and no participles, so forms like willed or willing are impossible for the auxiliary, but exactly what the main verb allows.
The noun is a different word again: free will names the power of choosing, so it never takes a sentence slot as a verb. Learners can safely set it aside when classifying the verb form.
Practical check: if will sits before a bare verb (She will come), it is the modal auxiliary. If it takes do in questions or carries endings like willed or willing, it is the main verb.