If you suspect a player might be "sandbagging," you can use this table to determine the exact odds of that player achieving a particular round.

For example, the odds of a golfer with a course handicap of 14 beating their handicap by 8 strokes are 1,138 to 1.

In other words, if the average golfer plays 20 rounds a year, it would take them over 56 years to achieve such a round just once (1138 ÷ 20).

This isn’t to say that great rounds don't happen,  they should absolutely be celebrated.
However, this table can help you gauge how often such performances are likely to occur.
If they happen frequently, it might be worth reassessing that player's handicap index.