There are those who proclaim that the block/charge call is the most difficult in basketball. Consistently getting traveling rulings right at the end of a dribble with rapidly moving players in congested areas is much more difficult. That situation also occurs much more often than the block/charge situation.
Fundamentals.
To judge traveling accurately, you need to see both the ball and the feet. Consider the uncontested layup, for example. The player will leap and catch the ball with both feet off the floor. One foot then lands and the player takes a long stride, landing on the opposite foot, and leaps for the layup: a rhythmic one-two sequence. The first foot to hit the floor is the pivot foot, which can be lifted legally before releasing the ball on a try.
Now consider the case in which the player catches the ball to end the dribble with one foot on the floor. That foot is the pivot foot and cannot be returned to the floor before the ball is released. In that case, the normal one-two rhythm described above is illegal. The key element is the position of the feet at the instant the player catches the ball to end the dribble.
Situations.
The Eurostep is a favorite move these days. It occurs when a player ending a dribble steps at an angle to one side of the straight path of his or her dribble, followed by a leap off that foot to the other side of that straight path, landing on the opposite foot and normally rising for a try (or a pass). If one foot is on the floor when the player catches the ball, it is a traveling violation when that foot returns to the floor.
The spin move is also popular. Focus on the foot that corresponds to the direction of the player’s spin to see if it returns to the floor before the ball is released. For example, if a player spins to the left, focus on the left foot. Often the pivot foot must be replanted to gain the balance needed to rise for the try.