Normally I would say "do a chamber cast, and then you will be sure".

In this case, there is a much more simple step one:

Look at the muzzle. Is that a 9.5mm hole or a 6.5mm hole?

THEN cast the chamber to be sure.

M1910 is 9.5X57. To change it to 6.5X54, they would have had to change the barrel, change the bolt (the chamber rim is a different size), change the magazine... and then re-proof the whole thing.

But a big 9.5mm hole in the muzzle would be your first clue that it probably wasn't changed. A smaller hole would make me look at other things.