I can tell by going into my garage to my workbench, looking at it sitting there among my tools and stuff, and getting a warm fuzzy feeling that makes my happy. Then I know that I have an HPI Baja 5B - there is no other!
Then I take it out, run it and do something stupid that results in having to buy genuine HPI parts (or even more expensive aftermarked upgrade parts) to repair my genuine HPI Baja 5B.
It's hard to tell without having the two side by side. The clone parts have more of a pronounced mold release line. The HPI plastics don't have much of the mold lines, if any. The glossiness of the two is pretty much the same these days.