How does one validate an idea for a SaaS/MicroSaaS?

I start sending out a form, or an open question on Reddit (you should check out r/SaaS)

Market research, survey, etc.

Have 1-1 conversations with 20-40 people who you think are facing this problem and ask a ton of questions. Reflect on the patterns you see. The most important thing is to deeply understand the problem. The best book I found for validating ideas is the Mom Test by Rob Fitzpatrick: http://momtestbook.com/