The test here is unusually clean: "passed" is only ever a verb, and "past" is never one. If the word is doing something in the sentence, it takes the double "s." Everything else — time gone by, going by something, the previous version — takes "past."
The past tense and past participle of the verb "pass." Always a verb.
A noun, adjective or preposition. Time gone by, or movement beyond something. Never a verb.
Quick way to remember
Ask whether the word is an action. Actions take "passed" — she passed, the motion passed. If the word describes time, position, or something previous, it takes "past." When in doubt, try replacing it with "went by": if that works as a verb, use "passed."