when I was a building a few fences, I took a pair of binoculars, and sat them on top of one post, and sighted through to the far post, and I had a helper, (we took a piece of twine 100 feet long, or so, and and when the water filled bottle came up to the post, he stopped and via the binoculars, he help up a taller post, and when it was in line drove a 3/8 steel rod, (electric fence post), and we did the 1/2 mile that way, after a few posts were in my helper could nealy line up on the driven posts, and was with in a inch or two normally, tried to keep the fence with in and 1/2" of straight, we used the small two way radios and hand signals, the hand signals were actually easier to use, then I went back and drove wood posts, at the steel posts, stretched out my barbed wire and when back in a drove steel Tee post, in between the wood posts, I think that fence was with in an inch of straight,
ended up building about two and a half, more miles of fence later that year using the same method,