There are all sorts of wonderful sewing gadgets out there, but the one I rely on for all my projects at some point or another is my trusty seam gauge.
My favorite type is a little multi-sided gizmo that gives a variety of measurements on both sides.
I got mine at a local fabric shop, but it can be got on Amazon here for a good price. In fact, get the two pack. I have at least two and they tend to escape right when I need them, so having a spare can't be a bad thing!