How do I tell, looking back at a game afterward, whether a sacrifice that worked was actually sound or just a blunder my opponent happened to fall for?
These answers restate the rules implemented by our open-source engine, which follows the WXF laws and is regression-tested against the official 110-diagram repetition casebook. Where an answer refers to a specific position, that position lives on the linked reference page, where the build verifies it against the engine before publishing.
Direct answer
Replay the sacrifice against your opponent's best available defence, not the move they actually played. A sacrifice that only works because your opponent missed a stronger reply was a blunder that happened to succeed, not a sound combination. The honest test is going back to the critical position and searching specifically for the defence you would least want to face, then checking whether your idea still holds up against it.
Why the result alone is not proof
Winning a game after a sacrifice feels like confirmation that the idea was correct, but the opponent's actual moves are only one branch of what could have happened. If a calmer defence was available and simply was not found over the board, the sacrifice was objectively unsound even though it produced a win on that particular day.
How to actually check it afterward
Go back to the position right before the sacrifice and spend real time looking for your opponent's toughest possible replies, not just the one they chose. If every reasonable defence still loses, the sacrifice was genuinely sound. If one particular defence holds or refutes it, you have learned something valuable about the idea, and about what to expect if you try it again against a stronger opponent.
Related questions
Does it matter if I win the game either way?
Not for the result of that game, but it matters enormously for whether you can trust the same idea next time.
Is a sacrifice still worth playing if it is not fully sound?
Sometimes, as a practical try against a weaker or short-of-time opponent, but it should be a conscious choice, not a mistaken belief that it was objectively correct.
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