On my last few pole-barn projects, I've welded four drops-pegs to a piece of 10" x 10" or so plate and set it in the concrete for each post. Then I get some tall L-brackets made by my local fab shop and weld them to the embeded plates one the concrete's set. Your brackets need some height and heft to resist moment on a pole-style building and need to be of sufficiently thick steel. The little saddle-style brackets you get at building supply stores don't cut it. A local fab shop can make the brackets pretty cheaply - last ones I did for a 24' X 70' pole barn ended up being $18 each.
The handy thing about doing it this way is you have some flexibility to shift the brackets around a little bit to get them all in-line before you weld them down. Much easier than having to line up all the posts perfectly when setting them in the ground. If you're using 6X6 posts and the 10" x 10" plates mentioned above, that means you have up to 2" in every direction to move them around and get them perfectly in-line. I usually use footing-tubes or sono-tubes, then pour a pad around them later (if at all). But you can do it on a pad as well, as long as your pad has a deep enough grade-beam beneath the posts.
I don't have a flat surface to write on right now so this is a pretty crude sketch, but something like this is what I do (click on link below). Brackets are usually turned 90 degrees to what I have shown. I show on a concrete grade beam, but the same idea applies to using footing tubes.
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