Playtest and know what to look out for. Ideally you want to test if the focus you decided on earlier is getting through to your players. Ask them what they think your game is about. Maybe it’s something entirely else, maybe they have now idea. It can also happen your game is not fun at all. In any case, you will find problems while playtesting and that’s great. Decide what the biggest are, try and fix them, and playtest again.
Make sure you playtest with players you actually intent your game for. Testing with friends or family will only bring you so far.
If all goes well, you’re game is getting better over time and problems become smaller. Now is the time for the ultimate test: playtest it with your mother. It probably won’t go as smoothly as you’d hope, and you’ll find your game pretty much still sucks. Most likely your sweet mother has not played many games and won’t be able to get far. You’ll find you’ve made many assumption about the player’s knowledge to play your game. If it makes any sense for your game, testing like this is a great way to find problems and make your game more accessible.
For Circles I wanted it to be as accessible as possible, anyone should be able to play. Showcasing at events also helped out immensely. It’s surprising to see the variety of people that go to these events (or tag along). You’ll get so much valuable feedback out of it, although this only covers the first 10 minutes of your game. Which is super important, but won’t predict the success of your entire game.
As for playtesting, questions I would look out for were: Is there enough feedback? Does the player know what’s happening? Do they know what to do at any time? Where do they get stuck and is that a bad thing? What do players like? How can I enhance that?