Weight obviously but if you don't have 2 HCKs it might be hard to tell, as the HCK single is heavier than other singles. I think the color of the embroidery on the sleeve shows. I had a blue standard single and blue comp single. The letters were red on the standard, and gold on the comp single. But the comp single was a bit heavier and much softer and more comfortable than the sand paper-like standard. The unbleached standard single has brown letters, the white absolute has black letters.
I've seen white HCKs with green letters, and one with blue. I'd guess that one of them is a single and one a double but I'm not sure which is which. I was watching an HCK on ebay today, too. The guy said it was a double but the pics showed an unbleached single(only the single can be unbleached). The letters were brown, and next to a white belt appeared to be darker. Hope this helps, buddy.
This one:
HOWARD KIMONOS MMA BJJ GI!! HIGH RETAIL AND HEAVY DUTY! - eBay (item 140379978390 end time Feb-06-10 19:46:52 PST)