A character can accurately judge another's skill score by observing the character using the skill and by then making a dice roll using his/her PE + the same skill as the other character is using (but including any specialism bonus in Judging another's skill). The difficulty value is equal to the other character's skill score. If the character fails the dice roll, but is close (i.e. within five points), he/she will know the other's skill score within a five point range (i.e. 0-4, 5-9, 10-14, 15-19, etc.). If the character fails by more than this, he/she will simply have a feeling that the other person is better at the skill than him/her!
For example, Roger is an expert swimmer with a Swimming score of 5 and a PE of +0. He does not specialise in judging another's Swimming skill. His basic value is 5+ 0= 5, which gives him a certain result of 5. He is trying to judge how good Ariana is at swimming. He decides to risk two dice and so rolls 2D-1. He gets a 2 and a 5, giving a dice roll result of 6. In fact, Ariana is a competent swimmer (skill score of 2) so Roger judges her skill score accurately.
For example, Urlik is a good climber with a Climbing skill of 3 but no points of specialism in Judging another's skill. He has a PE score of +0. His basic value for judging another's climbing skill is 3+0=3. He sees Ariana climbing (she has a climbing skill of 3). Urlik chooses to risk two dice giving him 2D-3. He rolls a four and a one, giving a dice roll result of 2. Since this is lower than Ariana's skill score (but within five points of it), the referee tells Urlik's player that Ariana's climbing skill score is in the range 0-4. In fact, Urlik's player can work out that Ariana's climbing skill score must be either 3 or 4, since if it were 2 or lower he would have known it exactly.