Your students have trouble writing a thesis, don't they? (Yes, they do.)
Let's assume they're already outlining their paper--they have a thesis (of whatever quality) and a few supporting points.
Here's a quick way to assess where they're at.
Materials: One 3x5 card per student.
Have students write their thesis on the blank side of the card, and write their three (or two or four or...) subpoints (summarized, obviously, in a sentence) on the other side.
Then, they trade cards with a neighbor, letting them look only at the subpoints. The neighbor should attempt to guess the thesis without flipping the card over, and then talk to their partner about the differences when they finally read the "real" thesis. This helps the writer...
1. Clarify the meaning of their thesis for themselves by explaining it to their partner.
2. See if their subpoints really link to their thesis.
You can then collect the 3x5 cards in one easy stack, and try it for yourself, reading their subpoints and seeing if their thesis matches up.
It works. Try it.