Here's how to tell:
Verb: When "run" describes an action, it's a verb.
* Example: I *run* every morning.
Noun: When "run" refers to a specific instance of running or a series of events, it's a noun.
* Example: She went on a *run* around the park. (Describes a specific instance of running)
* Example: The *run* for president was a tough one. (Refers to the series of events of the election)
Let me know if you have another word you'd like to explore!