I'm using these as a guide..
http://zhome.com/Classic/CommonZRust/RustPrevention240Z.htm
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Starting at the front of the car. 1. Radiator support, lower. 2. Where the headlight extensions and the fenders meet (not important, I have spare fenders) 3. The front of the hood, especially on the inside front lip (I have a hood) 4. Passenger side fender well, under the battery box (As long as the firewall isn’t rusted out, it can be fixed) The less the better though. Anywhere on the firewall is bad. 5. Passenger side frame rail under the battery box and area where main wiring harness exits the firewall just above the rail. 6. Both sides of the car, both frame rails underneath the floorboards. These are usually squashed, if they aren’t rusted through near where the firewall steel and the floorboard steel are joined together (right under your heels when you drive), that is ok. Aftermarket rails are available. 7. Floorboards: I have yet to see a Z without some rust underneath the pedals and on the passenger side as well, where the firewall and floor meet. Try the old scratch it with a screwdriver trick. If it goes through, that is bad!!! 8. Underneath the Cowl between the windshield and the hood ( wiper arm area) 9. Around the front windshield 10. Around the Ttop seals or side glass 11. Directly behind the seats, the rear end assembly bolts to the floorboards. There is usually some rust here on the floors and there, normally close to the rocker panel. 12. The rocker panels themselves. 13. The lower door seams. 14. The rear Fender well area: Front corner at the rocker panel. Underneath the lip over the tire 15. Behind the rear quarter panels, the inner panels like to rust out. 16. Around the antenna. 17. Around the rear hatch seal areas 18. Around the tail lights 19. Above the muffler, the floor board of the rear deck may be rusted through 20. The spare tire floor area. Easily replaced with sheet steel. 21. Passenger side inner rear quarter panel, next to the spare tire floor. 22. Inside the rear hatch itself, the bottom likes to rust. I have spares. 23. The floor of the hatch area, a visual inspection usually shows up bubbling paint if it is rusted through.
From http://www.zcar.com/forum/13-car-talk-forum/245562-most-common-rust-spots.html