Do you really need to check your phone to notice which tables in the cafeteria are gonna get covered in sun while you’re sitting there?
You can usually notice a bit of a shift while you’re standing in line to get your food. It’s not rocket surgery.
I went to high school in the 90s, and that’s how I figured it out, and I didn’t mean to check any sources. I just looked when I walked in, looked when I got in line, and looked when I got out of line. Three points of reference, I noticed the shift, and I could figure out what table to sit at. Easy.