I'd go with:
Min 100 posts
Must be member for 1 year+
Active member (for dave to interpret as he wishes)
That will wead out the people who join just to enter, and those those that go back to their inactive account and post 99 new posts to get enough (still doesn't count as active).
Game of skill wouldn't never work, some are experts at repair, some in mechanical, some in software dev, some in circuit design, etc etc. How would you pick questions easy enough for everyone to know, but hard enough not to be defeated with google ?
I don't think there is anyway to 'test' people's likelyhood to keep an expensive scope (no one in this thread has said what it is exactly). On paper I wouldn't be someone that would keep it, but the reality of having a project that needs a decent scope (and not having anything at all), and having not won anything in the past 20 years, mean I would chain the thing to my bench.
Only other thought it to award the prize to a project (if dave likes the project you submit), you get the scope. That would show need for said piece of test equipment.