If I can see triangle-like patterns, I imagine these resulting from a slice through a cone. I then imagine which way the cone must lie in relation to the surface of the wood based on the direction of curvature on the endgrain. That tells me which way the grain is pointing, and which way to plane. (Does this make *any* sense at all?) Can post pic if there's interest.
Then there's all the other techniques. Usually still get it wrong 50% of the time, which is the same success rate as chossing a direction at random (!). It's a bugger of a thing.
I'm in the middle of making a box out of mahogany, which seems to conspire to have the grain running all over the place, even though it looks straight. I might have to break out the scapers.
Those are my principles, and if you don't like them . . . well, I have others.