Guessing the disease
William Branham admitted in 1950 that he often guessed at the disease. We would guess that perhaps he got better at guessing after years of practice.
William Branham's admission
And said, "You'll feel the results of it" (Now, I call it vibrations.) "upon your hand. You'll become familiar with that. And you'll tell people all of their diseases, and what they've got in their body. And then if you'll be..."
Now, when I was in Texas the other time, that was in operation. Is that right? Any of you was in my meeting before, raise your hand if that was in operation? And if this... Now, you people raise your hands, all that was in my meeting, and know that was in operation. Not perfect, because I--I guessed at the diseases a whole lot. See? Because I didn't know how it felt. I'd feel a funny feeling; sometimes female trouble and cancer. I couldn't detect it; it sounds so much alike, was--felt so much alike. "Do you see the visible results on my hand?" And so forth.
That, which I guess, there's many of you here tonight, know that how it come to pass. First was to discern the diseases... kind of diseases. Then I did that till it got to a perfect way where It never failed. Then It started in to telling the people the secrets of their hearts, the things that they had done in life, all the way down through life's journey; which many of you might stagger at that now, and get it wrong. 
What the Bible says
There is no guessing allowed in this scripture.
The mark of a true prophet - Daniel states, “I will show unto the king the interpretation.” To prophesy is not merely to foretell remote events: to prophesy is to disclose the unknown—to unveil mysteries. False prophets are a curse; a true prophet is an immeasurable blessing. Guesses at truth are untrustworthy, deceptive, perilous. Real revelation is a safe anchorage for the soul.