How do you validate the viability of a community?

Hi people - I need ideas of how to validate a community before going into the full scale build.

I've done a SWOT analysis and market research as best I can but with a unique enough niche it's not really comparing oranges to oranges so I can only hypothesise.

But, what are some good ideas of testing the viability of a community before actually launching a platform for them to connect? Do you need to build a social media following first? Or do you need to collect emails before launching? Or do you launch with a MMP barebones forum & see who uses it?

Thanks!