That's probably because the process you would follow to conceive your test plan are the same no matter what medium you are testing, for example:
- Define the goal of your testing, e.g. you want to benchmark the
- Define what you want to measure, e.g. speed and errors (quantitative), emotions and opinions (qualitative).
- Perform the testing.
- Analyse the results very carefully.
- Create a report with findings and recommendations for your stakeholders.
Those stages (at a high level) are complete agnostic to the medium under test.
So in a paper form you are interested in the labels and the instructions (do they make sense, are they appropriate to the users mental modal?), the organisation and sequence of fields (is it logical and matches what people expect). You will also be interested to see if the size of the fields is enough to capture the data people will be writing into those fields.
You can use a digital camera to record what people do and say.
You will note that everything I just just described for a paper form are the same for electronic forms too.
Ultimately you just want to to make sure your paper form works effectively and efficiently with the people who will be using it.