There is only one 100% reliable solution that most foreigners are unaware of: the huissier de justice, who is a private judicial officer with the ability to personally attest to a situation and whose constatation is the final word in a court of law.
You can pay a huissier de justice to verify that the landlord is legit, to visit the apartment and certify that the pictures are not misleading. No one here would ever rent an apartment without having a huissier personally inspect the place and report any apparent fault that the landlord didn’t mention in his ad. This is particularly important because no landlord can hold you responsible for degradations if they were reported by the huissier before you signed the lease. Without a “constat”, the landlord knows you would not want to navigate through the esoteric french legal system and are most likely to drop your deposit than challenge his word. So many landlords here prey on unaware foreigners it’s a risk you just can’t afford to take.
Now, why is the huissier so reliable? There is a limited number of licenses available so no one can create an office if the government doesn’t think it is necessary. It is extremely rare. Because licenses are in very short supply they’re also very expensive, far out of reach of most law students who have to work as apprentices for years in the hope that their boss will sell his license to them when he retires. In a sparsely populated rural area a license can cost several hundred K’s. In Paris it’s in the millions.
Then, assuming the aspiring huissier can finance 10% of the acquisition and find a bank to loan the remaining 90% (and banks don’t loan money easily here), a long and harrowing confirmation process begins, because the profession is tightly controlled by the legal system (and also because france and bureaucracy go together like surrender and white flag).
Bottom line is: an huissier will never risk losing his license unless he stands to make far more money than his office will bring in his lifetime. I hate to say it, but in this country, as a foreigner, the only person you can trust is the one who can’t afford to screw you over.
While in france, make sure you have an huissier’s hotline number on speed dial.