Inside shopping carts lie clues: clues that can give an insight on the success or failure of a product/brand and the extent of it.
Your product is a sure hit if you see customers stocking up on it for no evident reason. Some examples of such products that I can recollect from my own experience: Maggi, Lacto calamine, Dabur Gulabari. Neither of these brands show signs of weakening, nor are they emergency requirements, yet the consumer wants to have them handy. The fear of running out of these products is reason enough to stock up.
The reasons could be many – brilliant product, ease of use, great service.
Bottom-line: If you have your customers buying the product in bulk for no apparent reason, you have done something very right .