That would be the Baggie Test. Full article here,
The Baggie Test - How to inspect for above surface bonded contaminants
I know I've shown this on My Classic Car with Dennis Gage on both car paint and window glass but at this moment I'm only finding this episode but the process is the same...
Claying doesn't always remove 100% of all contaminants. This is mostly noticeable if you do the baggie test AFTER claying.
This is why if it's important to me and the car I'm working on to ensure 100% of the contaminants are removed I'll use the Flex 3401 with a Medium grade Nanoskin Autoscrub pad and lots of lube. Just posted this to another thread,
The paint on the above car was mechanically decontaminated using a Medium grade Nanoskin Autoscrub pad on the Flex 3401 and then machine polished twice and then machine waxed.
How to use the Flex 3401 for Show Car Detailing
Besides detailing clay there are these options,
Nanoskin Towel
Nanoskin Wash Mitt
Nanoskin Autoscrub pad like you see above. IT can also be used on tools like the Porter Cable 7424XP etc.
Nanoskin products
My personal practice is I don't use Nanoskin products unless I'm already planning on doing at least ONE machine polishing step afterwards to remove any potential marring for the decontaminating process.