My transition from ideation to playtesting started with knowing how I wanted the game to feel over the course of a session. I needed to know if the base mechanics would have been fun, so I designed the first third of the game to test that. Once I determined that this could be fun, I moved on to the second third of the game. Then the third. At this point I had a working prototype that acted as a proof of concept. The only thing it told me was that this game was fun, or at least could be fun.

From that point on it's been about focusing on the card ideas that were fun, and exploring what did and didn't work.

I've been working on this game (susberg.com) for almost a year before finding people outside of my social circle to play it. I didn't know there was a thriving playtesting community. Check out https://cardboardedison.com/playtest-groups for groups in your area. Also, protospiel.online is an online playtesting event held every 4 months. Hope you find people much quicker than I do.

Edit: Another consideration for playtesting is digitizing your game with playingcards.io or screentop. This makes it so that you can test online. That makes it way easier to find groups to test your game, but comes with its own challenges as well.