Look out the window
A rule that EVERYONE should follow is
WHEN YOU SEE A RUNWAY AHEAD--CHECK WHEELS
Checklists do not always work, and it is possible to forget the whole pre landing checklist altogether!!!
It makes sense to have something totally separate from the normal checklist to trigger a gear check. And use it regardless of what type of aeroplane you are flying at the time. Make it a habit. And that habit will save you on the day when you are tired, the weather is bad, traffic is high, and the tower operator is cranky. Habits are powerful, and automatic.
And the man who has just landed wheels up does not need anyone to tell him. He is probably totally devastated, and considering his future in aviation, thoroughly disgusted with himself.
The fact is, this painful experience has probably made him a better pilot. He now knows that it CAN happen to him, if he lets it.
Criticism is inappropriate, and un necessary.