How to detect if a message is sent in Slack via the Slack Python API?

I'm trying to figure out how to do this and I've finally given up after attempting to checking the documentation, tutorials, and reverse-engineer some other people's bots.

Here is a small bit of code I tried to use to test it:

import slack
slack_token = 'xoxb-***-***-***'
rtmclient = slack.RTMClient(token=slack_token)

@slack.RTMClient.run_on(event="message")
def test(**payload):
    print("I'm finally working...")
    
rtmclient.start()

What am I doing wrong? How do I fix it?