One sure sign is if people ask for it. So far I've only had 1 request for merchandise (a t-shirt), so perhaps it was only a 1-off. But, 1 person did ask so perhaps if I actually put up a store people would buy something? After I had that request a lot of people told me to put up a store, but I haven't (for reasons irrelevant to this thread). Another sign would be a spike in website visits, fan mail, etc. Anything that demonstrates that you have fans that might actually spend money if you offered something.