Well, I wouldn't say "always".
What I meant was that the end that looks as it has been connected to the intestine seems to be more dark and dense than the rest. If it is "always" so, I don't know.
Mucus can be dark and dense too, but it is still different.
Basically, when You see a rhizome You will know it.
I don't collect or photograph that, but maybe someone can provide a more detailed picture of a typical rhizome?