Final answer:
Sara Lee would gauge the success of a new product at the Product launch stage, following market introduction and measuring actual performance.
Step-by-step explanation:
Sara Lee would determine the new product was a success at the Product launch stage of the development process. At this stage, the product has been introduced to the market, and its performance can be measured in terms of sales, market acceptance, and profitability. Up until this point, any prior stages such as concept testing, market segmentation, or product positioning are preparatory and speculative with regards to the product's success.