Rhizomes on a clumper (most anyway) will not go very far horizontally before they turn upward/vertical and make a new culm - often just a few inches before they 'J'. Runners will go horizontally indefinitely, culms will go vertical off them but they keep going. So if you have more than a very few inches of ___ shaped rhizome, you likely have a runner.
Brad Salmon, zone 5b/6 Southern Indiana
Winters -20 to -25C. Summers 30 to 35C , humid. 115 cm annual precipitation, frost free from May through early October. 259.3 meters elevation. Growing 150+ species. http://www.needmorebamboo.com/