This is a commonly asked question precisely because it is about a situation commonly faced by investment bankers. The generally agreed-upon answer is that the best methods for evaluating companies with historically negative cash flow are discounted cash flow analysis or relative valuations such as the price-to-sales (P/S) ratio. You can expand on that answer by noting you consider it essential, in looking at a company with negative cash flows, to conduct a thorough fundamental analysis of the company and the current state of its industry rather than simply looking at a single evaluation metric.