You pretty much have to eye it.
It wont be too hard, and the increments aren’t so small where if they are off by a spline or two, you wont see them.
If you don’t notice them by eye, then you’ll most likely notice it when you mount and start to ride.
If you really want to, you can use a flat edge to do it, or some other creative ways to make sure they are straight.
I would just eye them though.