There's ways to test for cleaning chemical contamination, but these test would have to be conducted with a well equipped laboratory & it's not exactly cheap unless you know someone who has access to one either through their school/work.
If pledge was used, that stuff leaves a waxy film on everything. Makes playing cards glide on surfaces like a puck on a air hockey table. I have a coffee table right next my tank, but it's fully covered except for the cutouts in the rear.
Swifter dust pads don't leave a slick waxy residue like a pledge spray would.
Breaking down the tank & running carbon is your only option at this point.
Good luck with your next setup.