I know less than you but...you would expect a stream of digits. For this purpose it may be worth getting, eg, a pack of soap powder where they print the UPC or whatever they call it beneath the bar code so that you can compare the stream of electronic information with the printed UPC. Having 'the answer' in front of you will result in additional clues.
I'd be guessing at ascii-hex 88, c1, 04, 00 ...but that raises a couple of interesting questions
-what about that single zero at the end...is that the representation of 00, or has a character fallen off at the end
-that c8J at the start, specifically the J. Maybe the J signifies a valid start sequence has been detected or something, because you wouldn't be expecting a j in a hex stream.
At some point, the manual for your reader has to be a help, and particularly when it comes to what the reader does with different encodings (maybe you get a code to tell you which encoding it is, maybe it is all 'transparent', I don't know and it probably depends on the reader)