✔ Press and hold the check button in the action bar to reveal errors while your finger is down. Any cell whose answer doesn't match the solution is shown in red.
Hold for about half a second and the check locks on (with a click, if Sound effects is on in Settings). Tap again to unlock. With a mouse, right-click the check button to lock it on or off.
This only applies to cells with a committed answer (large digit), not to pencil-mark candidates.
A check that shows a mistake costs the game its applause and full flash, just as a peek at a hint does: the win gets a single flash and a ta-da instead. From then on, a red ring marks the check button. While the check is locked on, placing a wrong digit counts the same. A check that finds nothing wrong costs nothing.