Locate the participle (not the entire participial phrase) and the word it modifies.

Dressed in a cheap suit, John looked like my uncle Earl.

A participle usually ends in -ed or -ing and is used as an adjective.

Which word do you think is the participle?

dressed

Dressed

The participle in the sentence is "Dressed," and it modifies the subject of the sentence, "John." The participle "Dressed" is part of the participial phrase "Dressed in a cheap suit."