I presume you're talking about the diagonal beam meeting the "north/south" beam (since I don't see anything that looks like parallel/perpendicular at a post).
You can use joist hangers - put the "north/south" beams on the posts, then use 60° joist hangers to locate and support the diagonal beam.*
Using joist hangers is a recommended practice anyway, since it makes a much stronger joint than through nailing into end grain or toe nailing them together. In many places, it's actually required by code to use them. You'd probably want to use post cap "joist hangers" (not sure of the technical term) to affix the beams to the top of your posts, also. You can get 6" by triple-2x caps that will work perfectly in this situation.
*I suggest this way because I'm not aware of any post caps that are designed to support a beam at a 60° angle to the top of the post. Of course, if you were to rotate your posts to line up with the angled beam, I'd recommend you do so, then you can set the diagonal beam in proper post-caps and use 60° joist hangers to hang the north/south beams from it. I'm not actually sure if joist hangers are made to support going the other way around, to be honest...