I had to do this myself a couple of weeks ago whilst building an observatory.
Here's a photo of my approach. I used a camera tripod with a makeshift plumb bob to ensure it was dead center of where the pier is destined to go, I ballanced a tube wrench on top to make it a little more 'pointy' and waited for solar noon (11:52 on that day).
Mark the position, draw a line, simple as that.
A week later I installed the scope and aligned it to the chalk mark on the floor. When I went to do a polar alignment at night I found that I was already within 20 arc second of true