Definition of nuts
When a player is said to be nutted, it means that his range is composed mainly of extremely strong hands. When a player is said to be nutted when he pushes all in preflop, we mean that he regularly pushes all in with AA and maybe sometimes with KK. His range strength is at its maximum. This is useful information to have when making hero folds. If you have a king flush on the river, you bet and your opponent pushes all in with a "nutted" range, it is probably a good time to fold.
Identifying nutty ranges with your HUD
To identify the nuts ranges of your opponents, nothing better than a tracker with a HUD. This way, we can evaluate the frequency of our opponents to extrapolate their range. For example, you make a 3bet preflop and a player 4bets you. In your HUD, you look at his 4bet % and you see that he 4bets 10% of the time. This is huge. Here, your opponent is not necessarily nuts. But if the same player 4bets 0.5% of the time, he's a nut. 0.5% of the time is the mathematical frequency at which you'll receive a pair of aces preflop. The chances of it being a nut are enormous!