I've always thought (perhaps, erroneously) that if there is some ambiguity in determining whether the given word is an adjective or a past participle, you need to look at whether the "source" of action is mentioned in the sentence. If the source is not mentioned, then it is an adjective. For example:
The disappointed boy was sitting on a bench.
Here, we don't know the source of the boy's disappointment, in other words, we don't know who performed the action of letting the boy down, Therefore, disappointed is an adjective.
But when the source is mentioned, then it is a past participle. For example:
The boy was greatly disappointed by his mom.
Here, we know the "source" - the boy's mom - therefore, disappointed is a past participle here.
However, this method doesn't seem to work in the following sentence:
One car drove away 10 minutes ago. Two more cars drove away 5 minutes ago. So now there are only 5 cars left in the parking lot.
Firstly, I thought that, since the source is not mentioned, it was either an adjective or an adverb, but the Wiktionary page doesn't reserve this meaning for the adjective "left" or for the adverb "left". It reserves that meaning only for the past participle "left", which is one of the forms of the verb "leave". So, most likely the word "left" in this sentence is a past participle.
However, the "source of leaving" is not mentioned in the sentence and it is not even implied, which means that the method that I've been using was wrong. But then I am really at loss in how to differentiate between adjectives and past participles.