Hi Pierre,
I am relatively new to FreePBX, so until the real help comes along: I am not sure what “working correctly” may mean to you. If you know the IP addresses of the phone and want to see if they are reachable, you could write a script to ping each one and send you an alert if no response. Just because they are pingable, doesn’t necessarily mean they are ready to take/make calls.
If you can log into the FreePBX GUI, you could bring up the Asterisk Info page (under Reports) which would give you a snapshot of extensions status.
HTH
Paul