All jokes put aside from my end, repros become more distinguishable when you put them side by side with originals. But of course, at first instance, this is something that someone who already collected original items would seem to be better equipped to do.
The weight and feel of the plastic is usually a quick give away, and the color too, as the plastics used are different nowadays anyway. Obvious moulding defects such as detail loss, or an obvious difference in a Cobra chest logo are also good indications.
As far as I'm concerned about Black Major's products, most of his stuff looks almost the same on the outside, but there are other subtle differences such as, holes in the plastic parts for the screws, or a peg on the Bat's arm that is larger than an original's, or the Inside of a part.
Unless mistaken, my general impression is that BM has shown in the past to have made an effort to make his repro stuff different from originals. So with time, you should be able to tell them apart from eachother.