A simple spark test should allow you to differentiate between steel and iron.
Past that, the only way to accurately do this is by removing a chunk and send it off for chemical analysis.
An Oxford Instrument? This one will NOT do what you need.
http://www.oxford-instruments.com/products/analysers/handheld-analysers/pmi-scrap-metal-handheld-xrf-x-met8000
Just to get someone 2 miles away with a simple hand held Oxford XRF devise was $300 the last time I called one in. It took 1 minute to turn it on, warm up and take the reading. It took 2 minutes to write the bill and me sign it.

Now this one will.
http://www.oxford-instruments.com/products/spectrometers/optical-emission-spectroscopy/pmi-master-pro
Last I checked a similar machine cost $35,000.
Unless some one in your area has a similar device, you'll have to send in a sample. To have some one deploy to your location is going to be VERY expensive even if they are located across town.
You have to have a chemical break down of all pertinent elements before you can start the number crunching for Carbon Eq.

The foto attached is one of the bases of arch span girders in the Frankfurt, Germany Rail Station. When I saw these, I wondered; HOW would one perform NDT to assure their integrity.

Ari,

WELCOME TO THE AWS WELDING FORUM!!

So, 'wrought' means 'to work'.  Until the mid to late 1800's it was used very commonly for lamp posts, axes, swords, rivets, trusses, handrails and decorative railings, and even warships, etc.  It was low carbon, malleable, and very easily welded.  More than likely this is what your 'trusses' are but you are correct, they 'could be' mild steel. 

Then, mild steel came on the scene.  This replaced much of the usage of wrought iron.  Leaving a term that has carried over into today which normally applies to railings (decorative or not) and other items that are not 'wrought'.

There is a big difference between cast iron and wrought iron.  Mainly, wrought is malleable and easily welded, not so cast iron.  And, a big difference between cast, wrought, and/or mild steel or structural iron. 

Breaking this brief info down to your query, a GUESS would lead to the conclusion that it would be weldable regardless of which material it is.  The question is, to what procedures?  Pre-heat, filler material, welding process, etc. 

I'm trying to figure out why a testing lab needs to know which Carbon Equivalent Formula to use?  They should be able to do a material analysis that would tell them what it is by carbon content as well as most other alloying agents added to the iron to make it what it is.  Carbon Equivalency is not as much for analyzing materials as determining other factors once you know what the material is.  Things like pre-heat in relation to HAZ control, hydrogen control, restraint issues, etc.  Seems like the issue is being evaluated backwards.  Find the material identification first, then determine which CE to use.  Not using the CE to find the material identification. 

Just my two tin pennies worth.

He Is In Control, Have a Great Day,  Brent