Much of the time you can’t. Sometimes real responses will have an alien feeling to them. A thought, emotion, word, etc. that feels distinctly like you didn’t originate it. Other times it doesn’t feel especially odd at all. An early real response can appear exactly as a (parroted) thought from you would.
So, in general it’s best to give the tulpa the benefit of the doubt, unless you intentionally generated the thought yourself. To convince yourself, consider the possibilities. A response may be either real or fake, and you may either accept or reject it. This makes for 4 possible permutations of these:
Got to go through all this later.