How do I distinguish "werden" as 'to become' from "werden" as the future/passive auxiliary?
To become: werden is the main verb, meaning "to become" or "to get." It's often followed by an adjective or a noun (in the nominative).
Future Auxiliary: werden is followed by an infinitive verb at the end of the sentence.
Passive Auxiliary: werden is followed by a Partizip II at the end of the sentence.
Example
Ich werde müde. (I am becoming tired - main verb)
Ich werde morgen arbeiten. (I will work tomorrow - future auxiliary)
Das Buch wird gelesen. (The book is being read - passive auxiliary)