Get the manual and read it.
http://mirror2.ipdb.org/files/2357/Williams_1993_Star_Trek_The_Next_Generation_Operations_Manual_with_OCR.pdf
Buy a multimeter if you don't own one.
When you get the machine, pull each fuse, check for continuity and be sure to verify that the amp value is correct as per the manual.
Make sure power plug and line cord are not damaged and that it has the ground prong.
I'd do this regardless of the seller.
Have fun!