The British government is advising UK citizens to carry proof of their last Schengen exit in case in wasn't properly recorded by EES.
This advice holds true for anyone from a visa waiver country that uses EES. (USA, Canada, Australia, etc.)
https://www.telegraph.co.uk/travel/news/foreign-office-warns-britons-carry-evidence-eu-exit/
Additionally, you can check your own status by filling in the form at this website:
https://travel-europe.europa.eu/ees/check-how-long-you-can-stay
It is an official EU website.