Swingle is easy to spot as a rootstock. It tends to be much lighter in color than the bark of the scion, almost always. Also, it tends to grow thicker than the scion, faster, so it quickly develops a "bench" at the graft union. All trifoliate and trifoliate hybrids do the latter, but of all of them, Swingle is the only one with grey-white bark.

There was definitely a bench on the graft. I didn't notice the color, but I'll look next time. Does swingle have thorns?

Well, there were some sucker branches growing on some of them, and I noticed a distinct lack of thorns. Which rootstock varieties lack thorns?