How test donor switch components

The easiest way to test them is to install them in a known good Switch. If you're planning on doing repairs regularly, it's an invaluable thing to have for a number of reasons. However, I do realize a known good device isn't a luxury everyone has.

You can also do a thorough visual inspection. Check for any tears in cables, or funny business if things aren't working after it's installed. Especially with touch screens, you may be able to see if something is amiss. Digitizers (touchscreens) can be really sensitive to minor bend, and the cables can also tear during removal, or reinstall. If you look at the back side, there is copper that follows the entire perimeter. This needs to be uninterrupted, I would also check for anything that looks like tearing in the cable area.

The image below shows how the cable attaches to this copper surrounding the whole screen and if any of these connections have become broken, the screen could malfunction.