There are ways to test for halogens such as chlorine, bromine and iodine. One such procedure is the Beilstein Test. This test helps detect the presence of halogens in plastic.
This test works on the principle that a material containing any bound halogens, or halogens in ionic form, will react with copper wire. When heated in a flame, a wire that contains halogens will produce a bright, green flame.
Take a hot copper wire and thrust it into the plastic sample so that the plastic melts and some of it sticks on the wire. Then put the wire with the bits of plastic on it on the hot flame.
If the test produces a bright green lasting flame, the plastic contains halogens. If there are impurities, such as fingerprints, on the plastic, this could produce a mild, green flame that will disappear quickly.