Granted the best answer was post #4.
hyphmngo, you will be well-served by that specific 8x loupe. I don't think you "need" a light box. You can use a window, computer monitor or glass from a picture frame over a light bulb.
The original gadget you saw is too expensive for the job and not enough magnification to see if the negative is sharp.
Please accept that the excessive discussion may be helpful to give you ideas of things you might one day want - may find at a garage sale or maybe you have lying around the house already.
But none of it is necessary.