1791 France 15 Sols

In all cases - it is better to leave a coin as is until the potential value can be estimated. Are there others in the group that have not been cleaned?

Actually you are correct about modern authentication requiring a forensic approach. As forgery methods improved the skills needed to detect a fake have also risen. When I started collecting in 1956 it was a much different world. In those days most counterfeits were fairly crude and there was no perceived problem.