What should I check for before turning on an OLD dish washer for the first time?
Check the water line connections. I would open the bottom access panel, and watch for any leaks while you run it the first time. Often the parts used are not of the greatest quality, including the solenoid valve that controls the water. 

On that note, make sure there is a manual shut-off valve, so you can turn it off quickly if there is a leak.

Beyond that, just make sure you run it through a cycle to ensure it's clean.