A seller or a previous owner should be able to produce that file, but if they cannot, it can be requested directly from the City of Saskatoon rather than taken on faith, since the property's own address is enough to pull the permit history.
Property tax records showing two dwelling units, and a fire separation report if the work is recent enough to have required one, add further weight, but the building permit and the occupancy sign-off are the two documents that actually matter to a lender deciding whether to count the second unit's rent. Expect to pay legal fees in the $800 to $1,500 range if a lawyer needs to help chase down or interpret any of it.
Without those specific documents, a two-family is really a house with an extra kitchen, not a two-family in the eyes of anyone financing it. Get your hand-reviewed valuation from Joel Dyck.