VAT and Company Registration numbers
French companies have both a company number (SIREN/SIRET) and a VAT number (numéro TVA). These numbers can always be checked by using the following websites:
Since realty agencies that have a focus on expats are often run by foreigner themselves, you might come across Belgian, Dutch or British registration numbers. These can be checked here:
It never hurts to just 'google' a company you don't know yet. If the company you want to work with has an office, definitely check out reviews on Google Maps or Facebook. For some professions you may find that yelp or stars of service are a more useful resource in France.
Wherever you look for these reviews, bare in mind that people are simply more inclined to leave a review after a bad experience than they are when they had a good one. Don't judge a book by its cover, especially if it consists of only one review.
Checking financial data
Again, we have to make the distinction between French and foreign companies.
Belgian companies: General data can be requested via both 'trends.be' (paying or use 24h free trial) or data.be (partially free). If you are somewhat familiar with financial reccords, you can always request the 'raw data' financial statements online from the national bank of Belgium.
Dutch companies: Financial statements and balances can be retrieved (after payment) from kvk.nl
Trustworthyness of websites
There is no magic bullet solution to check whether a website is legitimate or not, but you can always check the reputation of a website or reveil where a website gets hosted. Off course, a location of the host is only an indication and not sufficient to label a website as not worthy of your trust. But nonetheless, if the hosting company is a shady company in Bulgaria, it might be a flag.