There have been cases of bogus companies advertising on search engines and social media. Sometimes they have impersonated genuine companies. Often they prey on young or less experienced drivers who face heavy premiums from honest providers. You should be extremely careful if any of the following happen:
The quoted premium is extremely low.
They advertise a mobile phone number (a normal phone number is no guarantee that they are genuine either; these are easy to set up)
They want payment by any means other than credit card
They don't have a recognised website with clear contact information (although again these are easy to forge)
They cold call you by telephone or contact you via social media
They don't have an address where you can call to see them.
Remember, it's an old saying but still relevant: if an offer is too good to be true, it probably isn't. If you contact the company yourself via a reputable price comparison site it is likely to be genuine since they should all vet their providers before including them on their panels.