Vocational education aims at preparing students for their future as professionals; thus, students are often required to undergo a form of on-the-job learning. Assessment of professional performance during on-the-job learning is not based on single observations, but on prolonged participation at the workplace culminating in judgement by a workplace educator. However, little is known about "how" educators reach a judgement. This paper, therefore, aims at exploring how educators inform their judgements about students' professional performance. We consider the workplace to be a participatory learning environment, which entails that judgement about professional performance should be made in relation to the community students participated in and that the educator from that community has a central role. Our study takes an explorative approach based on interviews and uses template analysis to generate insight. Findings revealed that educators use multiple strategies to gather and process information about students to inform their judgement. Findings also showed that assessment starts at the first meeting, when the educator forms a first impression of the student and judges whether or not the student is suitable for the job. Educators at the workplace also showed that they are aware of the context in which they form their judgement.