I'm really just IDing from sight as I've grown both. The main visual difference is the size of the the lower petals. Other differences include life cycle (L. odoratus is an annual, L. latifolius, perennial), size and shape of seeds and L. latifolius, though sometimes called perennial sweet pea, is scentless. I'm sure there are other ways to tell them apart but I'm no authority either.
edited to add: L. odoratus is available in a much greater color range.