Activity Monitor shows you detailed % CPU usage by process, which may or may not be (I don't know.) limited to one core.

MenuMeters doesn't show you any where near as much detail, but it does show you how each core is being utilized. (Like so)

(I forgot to mention that you should select for "Percentage and Thermometer".)

Last edited by artie505; 12/26/17 10:37 AM. Reason: Add link
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