But it’s a weird thing to say. If your point was simply to give an example of that construction, there are much more plausible scenarios you could have chosen. And it’s probably also worth pointing out that in this sort of situation we often just say “If I didn’t know better, I’d think…”.
If I didn't know [that] you were telling the truth [at the time that you said that]
If I didn’t [already] know [that] you are telling the truth.
If I had not[already]known[that] you are telling the truth. which conveys the same meaning.)