Same way you "proof" a bubba'd milsurp or rebarrelled Remchesteravage. There ain't no cradle-to-the-grave bureuacratic handholding for doubles or Mausers in the USA.

But first you really need to define what you mean by "proof"...

See the Proof Loads post in double rifles for a discussion of the topic and then choose for yourself what method you feel comfortable with.