What is the smartest and most effective way to go about validating your idea and being absolutely sure of the market size and demand before writing your first line of code?
What approach do you take before putting months of your life into building a prospective software business?
How about neither!
With both of these, you are completely disconnecting yourself from the problem. You are looking for an easy way that allows you to spend some money and in the end you know: this is worth it or not. It's rarely how it works.
I assume you have an idea in mind, so ask yourself the following:
Try to talk to those five people about their struggles and your solution, and see how it is perceived. Once you've done that, talk to ten (1-to-1), then try and talk to 100 (through a community, Reddit, etc.)
Also, before you talk to those five people, be sure to have read the mom test (http://momtestbook.com/)