Play a whole game, or find three finishes. Your pick.
Two ways in. Play a full game against an opponent that is good enough to punish a loose move and not so good that you never win — or take three positions where one move ends the game and see whether you can find it.
The full game has all the real rules: castling, en passant, promotion, stalemate, the fifty-move rule, the lot. Drag a piece and it shows you where it can go, so you never have to remember whether that bishop was blocked.
You do not need to be good at chess for either one. You need to know how the pieces move. Everything else is looking at the whole board instead of the first thing that catches your eye, which is honestly most of what chess is.
The opponent is a small search running in your browser, not a real engine and not a person. It plays a decent club-level game in the middlegame and a clumsy one in the endgame, which is a fair description of most of us. Nothing is sent anywhere and no rating is kept — the scoreboard on the result page is stored on this device only.