If I were obliged to proof a smokeless chambered rifle for my own purposes, I would take the heaviest bullet intended for a working load. I would then take the maximum powder charge for that working load and increase it by 7%. As a back of the envelope calculation that would increase the pressure by 31%. (As a rule of thumb, pressure is proportional to the fourth power of the powder charge. Shows that you really don't want to go over max in a working load.) If the maximum charge is a slow powder case filler you may have to go to the maximum charge listed for the next faster powder and add 7% to the listed charge. Generally smokeless proof loads use a standard weight bullet with a higher or faster than normal powder charge. Use of a heavier than standard bullet is common in black powder proof loads.

If not a wildcat, the heaviest intended load would be at minimum the heaviest commercially available load now or in the past.

It should be noted, that if the pressure is too high you can have a brass failure and damage the gun even though the gun itself would be able to withstand the pressure. It's generally figured that the strongest brass may begin to flow if you go beyond 70,000 cup. Some cases will fail before that. A 444 marlin case should be fairly strong.

And tie the rifle to some tires, hide behind a big tree, and use a long long string to pull the trigger.