Look for pictures at militaryphotos.net or militarymorons.com compare them to Freefalls pics posted here.
[b]That said the pics are probably less of a giveaway than touching your stock.[/b]
The real stock is made out of a hard rubbery material (maybe it is resin?) it feels velvety to the touch and doesn't transmit thermal energy very easilly (if you put your cheek on the stock it will not feel very cold).
The material is a dark grey with some areas that appear to be dusty (whiteish and discolored on the surface [most likely in the 90 degree corners under the battery tubes and around the reinforced toe of the butt (the area that includes the sling points).]
The rubber buttpad on the real Crane is a soft sticky rubber, if you shoulder the rifle it should kind of snag on your clothes a little. (that is probably why some of the SF guys removed their buttpads, to eliminate that potential problem.) [personally I like the soft sticky rubber better].
If you remove a battery tube on a genuine stock and look at the closed bottom end you will notice that it is sealed shut with a different darker material (probably to simplify construction with a cheaper mold)
The latch on the stock will wobble, a lot (unless you did what I did and put a tiny O-Ring between it and the metal locking piece or added ugly rubber bands)
Also the buttpads on a lot of the genuine stocks have what looks like the marks that indicate molding flash was trimmed from the edge that meets the stock with an exacto knife.
That's about it.