You usually can tell just by studying the sound of the voice and the body language. But if you listen to your inner voice, you will certainly know. You'll notice if a spiritual entity lies to you in the same manner as you detect a fellow human who does not speak the truth. A spiritual entity never lies if you ask a question that could be answered with a yes or a no. In such a case, you can't call that a lie.
Spiritual entities do the same.
They could say things they believe in. Things that you would not consider to be true. They'll tell you whether they come from what you consider as the Light or not. It's a bit tricky this talk about what is true and what is not. Only you can tell.