Unfortunately whether or not the plant produces a tuber is about the only to tell. If your plant was full grown when you bought it, and the label does not indicate it is a tuberous variety or perhaps the seller may tell you, the only way I can think of is that when the daylight (or plant light) is less than l2 hours, the leaves of the plant get limp and start to drop - that is when the tuber starts to form, you should be able to feel it if you reach into the pot. The hard part is that there may not be enough tuber to notice unless the plant is allowed to go fully dormant, newly grown tuberous species may not produce as much tuber as you would like for the first growing season. Generally commercial tuberhybrida have a nice full tuber in the pot after dormancy. I hope this helps.