Test: does a helping verb pair with a main verb? If yes, the string is one verb phrase. In She will be late, will is the modal auxiliary and be is the main verb, even though it names a state, not an action.
Recognizing modal verb phrases
Modal + be as main verb: the modal comes first, and be names a state or condition rather than an action. In Will they be ready?, will is the modal auxiliary and be is the main verb, even in question order.
Modal + auxiliary chain: the modal stays first, but the next verb may be an auxiliary rather than the main verb. In will have been studying, the main verb is the last one, studying.
When the second verb is not the main verb
A classification trap: after a modal, the next verb is not always the main verb. In She will have been studying, three verbs follow the modal, but only studying carries the core meaning of the action; have and been are auxiliaries joining the chain.
How to decide when the chain is long: ask which verb carries the core meaning of the action or state, and treat that one as the main verb. Everything in front of it stays auxiliary, whether one helper or several. She will be late shows the short case: the modal is followed straight by the main verb, with nothing to skip over.