Some of the Charming China Cheapie knock-off spikes I've seen look really good.
If your Spike in question isn't an obvious fake, then you'll probably have to disassemble it (warning: this will violate your BM warranty and, depending on your circumstances, you will probably not be able to even buy factory service) and compare the internal construction with a known authentic Spike.
Even this way, it could be difficult to make the call. If the mechanism is obviously different, well then you know. But if our friends on the other side of the puddle copied the internal construction (which wouldn't surprise me at all), then you might not be able to tell this way.
Now we turn to tell-tale signs like machining marks. You see, manufacturers usually don't bother putting a cosmetic finish on the inside areas that won't show on the assembled knife. So, you'll see marks left by the machining process. Since CNC machines follow the same path every time and since the Chinese CNCs will be different that American CNCs, there'll be two different "fingerprints", one from BM's and one from their counterpart abroad's machine. The problem here is that I don't know if BM used a CNC on the Spike. They may have been done by hand. In that case, there may be considerable variation among authentic BMs.