how to measure the performance of marketing strategy

What is a good measure to evaluate the impact of a marketing strategy ?For example, does it make sense to increase the advertising budget? How will I know whether the increase in sales is due to this advertising and not because my competitor raised his prices? Sales is a bad measure of performance.
Can we link the effectiveness of marketing strategy to stock market returns of the company's stock? That way we get an objective measure of performance that is determined by the market and that is understandable to the finance managers who approve the marketing budgets.