How to detect if Auto-renewable IAP is cancelled?

I have done and tested everything that Auto-renewable subscription requires and everything seems to work fine.
But I am not sure how to check if user cancelled his monthly subscription after a few months, or even in a trial period?

For example, I want to show Ads again, if he cancells subscription, but,... how to detect it?
How to detect if Auto-renewable IAP is cancelled?
 
 
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