Melan and the more common Luten require knowledge of the inside to differentiate. I figured you had documentation but a peek inside works too.
The link to the page on your site gives a great look at the Melan system used here--great documentation.
I initially discovered it had Melan reinforcing because a portion of the arch had spalled, revealing the tell-tale steel ribs. That's the only way you can ever tell in the field. If the bridge is not deteriorated, its impossible to tell. However, later, I also looked the bridge up in the Historic Bridge Inventory and they had found some primary source documents that also mentioned the Melan reinforcing.
Hot off the press, I have a page for this bridge online which also briefly discusses Melan arches in general and has a couple photos that I think help explain how the Melan arch works. http://www.historicbridges.org/bridges/browser/?bridgebrowser=indiana/4thstreetbridge/
I'm a truss lover that is still learning when it comes to these arches, but I do remember Jim Cooper talking about this bridge in his notes as being a Melan design.
Nathan-- How did you figure out this was a Melan?