I have 2 sets of road bike pedals and was wondering where are they marked as to the size? 1/2 or 9/16?

Time to get out the calipers.

Having dealt mostly with the standard 9/16th pedals, anytime I see 1/2 threaded spindles they look odd and are easy to spot.

pedals usually aren't marked (except L/R or D/S) but the crank pedal eyes often ARE marked with the threading. And threading can change on old (French) cranks that have been re-tapped and re-threaded, so don't assume the marks are always true.

They probably aren't marked. But, you should be able to figure out very easily if it's 1/2, or 9/16 just by looking at it. If it seems small it's 1/2". Try threading one on to your crank. If it's too small, then you know its 1/2"