Eventually it will come down to intuition. But a good rule of thumb is solid lines are normal cuts, and dotted lines are V-cuts.
It’s best to have the 3D view of the pepakura file open while you are working on the foam so you can see how much of an angle to make your cuts. Remember you are following a GUIDE, you don’t have to cut exactly where it says. You don’t have to make every cut either. You can make as many or as few cuts as you want. What’s important is YOU think it looks right.