Depending when the refinish was done, if it was, the frame and other parts may be marked with a B followed by a diamond or an R-Bi inside of a rectangle, and a date (month and year stamped on the left side of the grip frame, for example, 2 60 would indicate the work was done in February 1960). If the Magnum was returned in the last twenty years there would probably be no markings indicating work was done.
Bill