Locate the participle (not the entire participial phrase) and the word it modifies. Dressed in a cheap suit, John looked like my uncle Earl.

Participles are used to modify nouns in a sentence - they are often used as adjectives. So, here, the whole participial phrase is dressed in a cheap suit, but since you only need the participle, and not the whole phrase, the answer is dressed. The word this participle modifies is John.

Who was dressed? John.