I don't see how examining grain will show lens distortion. A negative will always have grain regardless of sharpness or focus and grain is so small that you would never see distortion in it. And grain orientation and shape is unknown so you couldn't see distortion by inspecting grain anyway. And you would never see focus in it unless you were testing enlarger focus which isn't what you asked.
To see distortion I would create on your pc and print out a square grid of fine lines. Then photograph it from fairly close up taking five images with the grid in each of corners of frame and in center of frame, preferably without moving camera and don't change any settings. It would be better to use 5 grid prints so that you only need to make one photo of them.
Then also make same photo at a distance of at least 10 times focal length.
Then develop to a higher contrast than normal or better still would be have a lot of light on grids so that they are high contrast to start with and develop normally which would be a better representation of real world practice.
Inspecting shape of grid will show distortion in corners of image if there is any. A normal neg loupe should show this.
If wet printing use normal or soft contrast which will NOT remove edge detail of grain (printing high contrast will remove grain edge detail and create false impression of grain sharpness and emphasize grain but not actual neg grain. Finely spread grain such as in shadows will be lost if printing high contrast which is opposite of what happens if developing to higher contrast).
If you are testing zoom lenses then you should repeat photographs of grids at different zoom lengths to find where distortion is pronounced or not so one image at minimum zoom, one at middle zoom and one at full zoom. And do both fairly near, say 2 meters and then again at least 10X focal length distance.
If you are scanning neg to check it, then scanner pretty much destroys film grain (depending on scanner) and creates its own so grain in scans is a product of scanner and not directly film so you can't really tell anything from scan grain.
You would see the grid distortion though.
If its also focus you are testing then see
http://www.normankoren.com/Tutorials/MTF5.html
And perceived sharpness is also a function of edge contrast and not just lens focus. Increasing print contrast increases perceived sharpness and so does increasing neg development but that something different than increasing graininess of print.
And finally, most taking lenses are not designed to be flat field in the subject so the focus plane is not a real plane at all. I mention this because lens tests showing focus/sharpness/resolution falloff towards coners invariably don't mention this and don't specify whether they are testing flat field focus plane or not. With extensive testing you could determine the shape(approx ellipse) of fine focus but it would be a real pain to do it and is probably not necessary in 99.999999% of cases unless you are doing some very serious scientific measuring work.
MTF datasheets for lenses (if you can lay your hands on them) should show the distortion at different zoom lengths which is usually the case with zoom lenses.
Could you explain exactly why you suspect distortion in your lenses which would aid in suggesting best approach of how to proceed.