Use vectors to determine if points are @ right angle

Use vectors to decide whether the triangle with vertices P(1,-3,2), Q(2,0,-4) and R(6,-2,-5) is right angled.

I have a theorem in my book that says two vectors are orthogonal (perpendicular) if and only if their dot product is equal to zero. But I seem to be having difficulty turning them into vectors. Also, it seems that there should be 3 different sets of vectors that I could choose, how do I determine which are the best choice without graphing it out? ...and is there a method where that doesn't matter?