Knowing when to call "liar" is the most critical skill in the game. Challenge too early and you lose a die needlessly; too late and opponents build impossible bids unchecked.
The 30% rule: If the probability of the current bid being true drops below roughly 30%, it's often worth challenging.
Factor in your own dice: Always count how many of the bid's face value (and ones, if wild) you hold.
Consider the bidder: A conservative player making a high bid likely has the dice to back it up.
Should you challenge?
Current bid: "Six fours" — 15 total dice on the table
You hold 1 four + 1 wild ace = 2 matching. Need 4 more from 10 unknown dice. Expected: ~3.3. Bid is above expected — consider challenging.