Yes more valve seat unleaded fuel questions!

My engine serial number starts with BHM 1439 1XXXXX. While I was informed in
the past, via this newsgroup (thank you) that it is likely a rebuilt engine,
I still have not been able to identify whether or not my engine has the
hardened exhaust valve seat.

So, my questions are:

1. Can I look inside the combustion chamber,  via the spark plug hole, and
tell if an insert has been installed? Perhaps with a borescope?

2. Are there any other simpler way to tell.

3. If  I need to use additives, any recommendations, available in Paris,
France?

many thanks,
CM