It depends. If I am getting an amazing deal I just look to make sure the cosmetics are as described and everything is there....all the boards etc. this is usually when I buy a game that isn't working.
If I am buying a working game, I will check the switches, coils, displays, take a look in the cabinet, check for water damage, and look for acid damage or bad board repairs.
If I am buying a game from one of the many other pinball folks I know here in Texas or Oklahoma I just take the word at face value and don't even bother to power it up.
Ken