I have a 1 mm thick plastic sheet, the plastic is not opaque.

Some gas pockets, I would say some bubbles, are inside the sheet; the size of the bubbles is about 1 mm.

What methods are available to know the composition of the gas that is inside the pockets?

Methods can destroy the sample.

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    $\begingroup$ Gas chromatography, possibly even GC/MS. Sample a bubble using a small syringe and put the sample through a GC/MS set up to identify common gases. $\endgroup$ – matt_black Jul 14 at 11:56