How umpires check it
- Identify the batter-runner and every occupied base at the start of the play.
- Ask which runners were forced to advance because a trailing runner needed their base.
- Watch whether a force was removed by a trailing runner being put out first.
- On a caught fly ball, check whether each runner legally retouched their time-of-pitch base before advancing.
- Separate live outs from appeal outs, then apply any run-scoring timing rule.