Hard to describe but the SS's rhizomes are more white than yellow, irregular and knobby. They always remind me of skeleton fingers, kind of knuckle-like bumps at each years growth. If your Bamboo is mature Phyllostachys than they would be a good bit larger.

One explanation for the common name 'Solomon's Seal' comes from the likeness of a stamping seal that would used to impress into wax. I can almost picture that but don't know if its true.
Here is a site w/ a good pic of SS rhizome, even if you don't speak French it shows the characteristic 'seal' part up close (scroll down the page).

Here is a link that might be useful: SS rhizome close-up