A lot of fly rods have the weight printed on them. If not printed, the weight is usually available at the company's website. All my fly rods weigh 3-4 oz.
I've never seen a casting or spinning rod with the weight shown, though. You'd have to weigh them to be exact. You could always take them to the post office.
This seems to be an American thing. European rods generally do show the weight somewhere.