Any old magnifying glass will work or a jewelers loupe will do if you are holding up the negative. Loupes made specifically for negatives usually require a light box to rest on. A flipped prime lens also works like a 50mm.
Also if you sit at a table with a desk lamp, point the lamp downward at a piece of white paper. You can then hold the negatives up and look through it with the bright white paper as the background. It's a little easier than holding sheet and sheets up with you neck back.
I have used my iPhone with a blank white screen, from a new tab within the web browser with max screen brightness, to quickly scan negatives that are drying.
For my students I have a setup that I use to invert the negatives on the large screen iMacs we have. I lay the negatives on our large light box connect a web cam that has a long USB cord, and place webcam lens right against the loupe. I then invert screen colors on the Mac and use QuickTime and start a new movie recording. I don't hit the record button but use the video feed from the webcam. It gives a nice large screen size image that is a positive. Helps the students learn how to read negatives quickly and for others in the class to see the negative as well to discuss.