When you absolutely hate going back and forth. Everything has to happen on the first run or not at all. Have to pack up a car for holidays? 'Yup, i got them muscles - I ain't coming back for another load.'
When you deducted god and dismissed heaven and hell by when you were seven. Premises: 1) god is perfect, 2) god is sapient and acting in the world. Deduction: 1) only "everything" has potential to be perfect. It cannot miss a thing, it must consist of everything. All the good AND the bad. All the matter and antimatter. Every fiber in you and around you. Every speck of stardust. Every ray of light, every shadow and complete darkness. Every noble deed and evil thought. Everything. Only then it's perfect. 2) humans are sapient, dolphins and most propably few another alien species, too.You are part of everything. You are part of god. You are one tiny sapient part of this perfect system which is god.
Then again there is no need for heaven and hell in all this. It's all part of the perfect world anyways. There's no real need for prayers either, tho. Crom does not help.
So.. you know you are intp when you deducted that you are god.
Also: when this forum makes sense
Edit: also when multiple choice questions like "who'd you rather be" results in complete system shutdown while you ponder the lives of them options. Two options is ok, three is manageable, four or more fuck me up.