Regarding the first sentence your wrote. That's how you solve the problem. Graph the difference quotient for x=2 and h in [-0.1, 0.1] or some such suitably small region. It's quite easy to do in the computer. One just needs to know that this is what you need to do. I did not encrypt the problem. It's quite straightforward provided one knows the concepts.

If students are allowed to use Mathematica during a test then asking them to find f' using the definition of f is not helpful in determining if they understand the concepts. They, for all reasonable problems, just need to execute a single command:

Limit[ (f[x+h]-f[x])/h, h->0]

This doesn't really test their understanding. If one is going to allow students to use Mathematica on a test then the problems need to be adjusted.