DO you happen to have the paperwork for the cage-from the builder. I asked KIRK to provide me with a certificate for our cage. The document will tell you the material and sizes of tubing used. That should be sufficient to determine if yours complies with the DOM/Chromoly rule now in effect. If you don't then there is going to be some difficulty in determining what the material actually is.
I mentioned in the past that there are several ways to make DOM according to the US Steel handbooks. One version is pierced/extruded billet and does not have an ERW seam.

------------------
Grandpa's toys-modded suspensions and a few other tweaks
'89 CRX Si-SCCA ITA #99
'03 Dodge Dakota Club Cab V8-Patriot Blue gonna tow