Nothing is certain, but you can sort companies into "likely to gain" and "unlikely to gain." The P/E and P/CF ratios are commonly used for this, along with other valuation metrics. The basic theory of value investing is to find companies that are likely to give you a return.

Also, historically the stock market as a whole has outpaced inflation, so even just investing in randomly chosen companies should have gains over time (there is even evidence that a portfolio of randomly chosen companies performs about the same as a portfolio of companies chosen by analysts and advisers).