Best metric to give the clearest picture on cash available to re-invest in business?

I'm hoping I can phrase this question in a way that makes sense.

What metric would give me the clearest picture of the amount of cash a business is generating that could be used to re-invest in the business? I.e. let's say I only want to invest in companies that have tons of room for R&D, CapEx, etc — what is a screener I can do to determine such companies?

Am I overthinking it and the answer is just cash flow per share?