Since you are using a (day light - I assume) bulk film loader, a thorough inspection will be sufficient.

All of the bulk film loaders work in the same way, more-or-less.

If there was a seal or light absorbing material, a test would be in order. However, the design requires that the light is blocked with baffles. The film cassette mouth fits into the film loader feed. The whole cassette then fits into a baffled compartment which is sealed before you wind the film stock onto the cassette core reel. Baffles are preferable to seals to help prevent scratches and cinch marks from being dragged against another surface contact.

Check for chips to edges, cracks, and evidence of damage to the bakelite plastic case. Since bakelite is dense and brittle, damage will be visible and obvious. Be scrupulous with your examination under adequate light.

If there is no apparent damage to the bulk film loader baffles, hinges, cover, and cover cap, you're good to go.

Good luck.