This is the coverage map for Bouygues:
That link is for Champsac but just replace the postcode and town as needed in the box.
The other thing I found for the exact location of masts needs a smartphone app to be downloaded but it does work:
Since my first tests of my local signal it has markedly deteriorated and is down to 380kb download and 180kb upload, I also had an email from someone who has a mast very close to her house and has the Bouygues 4G data box package, she previously had a very good and very fast speed but over the last week the signal has disappeared and she can’t even load a web page this morning so I guess her local mast has a problem or here is a wider issue with their 4G service.
For me when I first started to investigate I could walk 1km up the hill from my house and get a 10mb 4G signal but that has now completely disappeared and is back to 3G with extremely slow speed (< 300kb). I’ve no idea what’s going on with their service as it seems very hit and miss whether or not will work from day to day and my French is not good enough to have a dialogue with them and try and get an explanation!!
PS I’m soldiering on with my Orange 1mb broadband service which is supposed to be 2mb, it’s painfully slow but at least it stays up most of the time.