56-56 or 56-50

[Sedalia Dave]

Is there an easy way to tell if an original Spencer is chambered in 56-56 or 56-50?

Been looking for an original to convert to CF but don't trust the online sellers to actually know the correct chambering.

I know the info is here somewhere but I can seen to find it. 

[levers]

Seems like slugging the bore is the only way.