The tag-question test is the best for finding subjects.

They saw the man who robbed the bank, didn't they?  vs.
They saw the man who robbed the bank, didn't he?

OK, your sentences have to be fairly short. You can't have everything.

The problem with using 'who/what' is that students have to monitor 
whether or not a "do" appears:

The ball hit the window.

What hit the window?
What did the ball hit?
-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Johanna Rubba   Associate Professor, Linguistics
English Department, California Polytechnic State University
One Grand Avenue  • San Luis Obispo, CA 93407
Tel. (805)-756-2184  •  Fax: (805)-756-6374 • Dept. Phone.  756-2596
• E-mail: [log in to unmask] •      Home page: 
http://www.cla.calpoly.edu/~jrubba
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