How do I unit test an Action Passed to another method

I am trying to unit test a method which has a line

_pollingService.StartPolling(1000, PollingHit)

Where PollingHit is a private method in calling class and signature of StartPolling is

public void StartPolling(double interval, Action action)

now whatever is inside PollingHit(), how do I test that ? Since responsibility of calling this method is on PollingService, but since that is not a class under test I should not be creating a concrete object of it.