Take the normal lens off your camera. Set the focus to infinity and open the aperture to the largest opening [smallest s/stop]. Look through the front of the lens and aim the rear [lens mount side] at the negative. Now you have a magnifying lens to use as a loupe.
35mm lens is best for that. :-)
OM-1n: Do I need to own a Leica?
Rolleicord Va: Humble.
Holga 120GFN: Amazingly simple yet it produces outstanding negatives to print.