If you do, do the police arrest you (I assume no)? What happens?

Your employer risks some (huge) fines, depending on where you live. Here in France, we have "Work Inspectors" who verify sponteously those rules − they can be called by employees themselves, too.

Of course, if you are self-employed, there is no way that those rules can be enforced… but it will be frowned upon by your relatives, because we know that it is wrong in the long term.

How do work inspectors do it? If they ask people and get told how many hours are worked it could work, but it's not like they could count hours..

(I moved to work in Paris this year, so am rather interested... though I believe my 'cadre' status exempts me from this anyway)

Usually they don't need to count. In general working hours are stipulated in the contract, so if they find someone in the office outside the hours they have stipulated in the contract the managers better have a good explanation.... Then, if they consider the evidence is enough there's an official investigation where they can search for emails/phone call logs/documents that attest that the employees were working or asked to work outside of "working hours" and a trial takes place. In my current workplace there are no such strict rules, but if management finds someone working noticeably more than 40h a week they'll want to know why that person needs more time than the rest to do his job. I guess that's as weird for non Europeans as working >40h a week is for them.