From AN8.67, this is ignoble:
Saying you’ve seen, heard, thought, or known something, but you haven’t. And saying you haven’t seen, heard, thought, or known something, and you have
From AN8.68, this is noble:
Saying you haven’t seen, heard, thought, or known something, and you haven’t. And saying you’ve seen, heard, thought, or known something, and you have.
Together, these two considerations create a powerful disincentive to make claims, simply because one might be speaking out of delusion and would therefore be speaking ignoble expressions. Essentially, not saying anything about personal attainments is wisest.
Yet even with this in mind, one might assert a personal experience--how can a listener know the truth of such?
You can get to know a person’s ethics by living with them. But only after a long time, not casually; only when paying attention, not when inattentive; and only by the wise, not the witless.
One comes to know the truth of oneself and others in deeds rather than speech. For more detail, study AN5.100, which discusses the claims of different kinds of teachers including the Buddha himself:
A person will be recognized by their own deeds.