Look for obvious signs of wear and tear. Test all the pots and sliders to make sure that they work properly (nice smooth operation, minimum noise). Also look at the heads, and if possible do a test recording (nothing too fancy. Try the old jangling keys and voice). Test all the transport functions to make sure they work as they should. If it's in good condition, and has been well looked after, you've got yourself an excellent little unit at an outstanding price.
Oh, and you can do more than just song-writing and demos with it.
Come join us on the Dark Side