You test for warpage like dantrcav said. Mostly it is the top plate. Tape a sheet of 1200 grit wet/dry to a very flat surface like a piece of glass, and rub the top plate across it. If you see areas that are not "abraded," keep going until the entire surface is smooth and shows signs of sanding.

There are a ton of posts on this subject on this forum, and I have a good article I can snail mail if it would help.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe