I forgot when I was replying this morning...
Red Solo cups and their imitators are
standard measurements at the trim lines. The lowest line is 1 oz. The 1/3 line is 5 oz. The lowest rib at the top is 12 oz. The full cup is 16 oz.
Starbucks cups and some Coke cups have marks on them. They're to show the level for various ingredients... fill ice to here, then soda to here. I did a quick Google for the lines on a Starbucks cup to see how much they were, but didn't find the answer I was looking for right off. I am sure it's out there, though.