Look for wear at the kickout (zone 5 on ES and under the ramp on WW. Neither of those area are covered with mylar, usually, and it shows on heavily played machines.
On ES, look very closely on the backglass as the paint is old at this point, and replacements are near impossible to find. WW translights are plentiful, however.
Look at the ramps, and I bet that you notice that they have cracks at the entrances. They run $200+ per set. Remember that ES has a spiral one to inspect as well.
Otherwise, the general issues with cabinets, playfields, functionality, etc.
Good luck.