That isn't true. If one system has you moving one way, and another system has you moving another way. I can tell the difference. LOL
OK, I'm gonna break with the majority opinion and agree with you. Maybe your problem was using Kenpo as an example, since it is a very broad and eclectic system. However, many arts are narrower in their focus and do not overlap so much. Archery and wrestling for example. I'm sure I could tell them apart even in totally unstructured application. LOL
Now a little more seriously, If you were to compare Wing Chun, TKD, and BJJ, most people could differentiate them even in a free sparring session. There is not that much overlap between their methods and objectives. For example I remember a post in which a BJJ practitioner on this forum commented on a video showing a WC guy sparring a karateka. He ridiculed them both for their inept groundwork. He noted that the WC guy passed up numerous opportunities to get the mount and submission. The fact that neither art has the objective of getting a submission was apparently lost on him. Might as well criticize the archer for crappy takedowns, or the wrestler for poor marksmanship!
Now of course if you are training for a common objective, there will be more overlap. For example, if you want to spar with MMA rules in a cage, and you want to be successful, you will apply what you need to, and the lines will be more blurred. Alan Orr's WC trained MMA fighters are an example: