How to measure self reported computer proficiency? Ideally with one item

For a usability test, I'd like to have a simple measure for computer proficiency, to be able to see, whether it has an impact on successfully handling the tasks.

How can we best measure that?

A simple question with a likert scale would be nice, but is hard to create. As I think just asking "how proficient are you with computers?" probably won't wield valid results as there is nothing to compare your proficiency to.