Well, you pretty well said when you said, "wait till they bloom"
, but, *generally speaking" .....with 'all things being equal 'as far as soil nutrition, the median rhizomes are smaller than the TB's, UNLESS the TB's are the older, Diploid types.
Another 'sometimes" clue is "PRR".....purple ringed rhizomes......just as "PBF".....purple based foliage.....can help ID *some* iris, a lot of the medians, especially the SDB's, will show a purple ring around the point where the foliage emerges from the rhizome. It's not usually readily seen while the plant is 'in the ground", but shows up pretty well when it's in your hand.
But, the only sure way.....wait until they bloom
