Jason,
i learned from experience. Ask teh owner to let you get it in teh air. then take a small pick hammer underneath and tap under the seat, framrails, and everything else you can get your hands on. if it sounds solid good if it pings, or you have metal flaking off, or the frame rails sound hallow, especially under the battery you may be in for some major cutting and welding. I thought my z was rust free. then 6000.00 later i had all new floor boards. If i could have done it over i would not have bought mine i would have kept shopping. just make sme mad that i could have put that money into the drivetrain if i had be patient and bought a rust free car. and on top of that i may not have gotten all teh rust... only timne will tell.
just my 2 $
stony http://photos.yahoo.com/bc/stonehac?d&.intl=us&.flabel=fld3&.src=ph