If you aren't clear on the cross measuring, measure from 2 to 3 and 1 to 4 in the first one, if they are the same, those points are "square". If you are confused on where exactly the point is on the frame you can measure from 2 to 4 or from 1 to 3 or 5 to be sure you are using the same ones.
But honestly, you see very quickly if you are wrong because the numbers are off a mile. These cross measurements on a frame like your truck should be within about a quarter inch difference from one to another.
You measure one, then cross to the other, when you find it's good, simply go to another point near by to confirm you are using a good point, once you have a few you KNOW you are good to go.