I am having massive difficulty in trying to decipher if the following sentence whether is a past participle in the passive voice or past simple in the passive voice?
The woman was robbed yesterday.
As Colin Fine said, "was" is the main verb, so "was robbed" is the simple past tense. Specifically, the "robbed" in "was robbed" is a past participle, not a past simple. In this case the "robbed" past participle was used as an adjective.
My advice for someone learning English is to identify all words that could be verbs. The relative location of the possible verbs can help with identifying their tense.
Typically, "was + [verb]" or any other "[to-be] + [verb]" implies a different form of verbs. My suggestion is to look out specifically for [to-be] verbs (is, are, was, were, etc.) to help with identifying the tenses of other verbs.
One helpful table would be "The Full List of Tenses" in https://www.grammar-monster.com/glossary/tense.htm.
EDIT: changed "past progressive tense" (was robbing) to "simple past tense". Thanks 3kstc!