Learning can take on a variety of forms from formal classroom, to experiential to one on one mentoring. What is the best way to determine if learning has occurred?
Traditionally
A traditional and oft used technique is testing. Whether written or practical the core objective is for the learner to prove their knowledge.
Unfortunately the context is artificial and this impacts the outcomes. For example if you are driving a vehicle alone, your responses and attentiveness might drastically differ from when the testor is sitting beside you.
Feeling nervous will definitely affect results. Cynically speaking the test results might only prove the learner is good at taking tests. Put them in a situation where the knowledge needed to be practised and the results could be catastrophic.
Is there a better way?
Demonstration of knowledge through execution ( like the driving test) resonates with me but is not always practical and the assessment of the results includes some subjectivity. (For example I would not recommend testing a surgeons skills on open heart surgery).
Japanese sushi chefs undergo a long apprenticeship which provides them with a great deal of practicle experience. The final test is preparation of Fugu fish sushi.
Fugu is a highly toxic fish that when eaten, if prepared badly, results in almost immediate death. The sushi apprentice eats the fish they prepared and if they live, they pass. Now thats a test.