Pulley Bearing noise?

My Starboard Mercruiser 377 has started making a noise. I am hoping it is just a one of the idler pulley or tensioner pulley.
What is the best way to pin point which one has the bad bearing?
https://youtu.be/QJH3VUYkNfQ
What is the best way to pin point which one has the bad bearing?

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2018 Cherokee 39RL Land Yacht (Sorry...)