The major way to distiguish a main verb and a verb phrase with a participle in a sentence is to pay close attention to how the verb functions or what element does it modify.
While main verbs express action, participles look like verbs but function as modifiers or adjectives, and they usually end in -ed or -ing.
For example, in "The smiling postman waved at the children", the participle "smiling" functions as a modifier, indicating what kind of postman was "waving" (the main verb expressing an action).
Another example would be "The meal cooked last night smelled good". Here, "cooked last night" explains which meal performed the action expressed by the main verb "smelled".
To conclude, while main verbs express or indicate action, verb phrases with a participle function as adjectives modifying nouns.
