I heartily recommend Kirkpatrick’s model:
Level 1 – reaction – ask the learners for immediate feedback about the course. How do they rate it? Did they find it useful or interesting?
Level 2 – learning – did trainees actually learn what the program was supposed to teach? Test them somehow to see if they can demonstrate the skills or knowledge that the course was designed to impart
Level 3 – behaviour – when they returned to the workplace after the training event, did they do things differently than they did before? Check to see if they are using their newly learned skills and knowledge ‘on the job’
Level 4 – results – what benefits did the organisation gain with these people now performing differently or better in their roles?
Success at one level tells you little or nothing about whether the program is successful at higher levels.
The fact that trainees enjoyed the course does not prove that they mastered the skills it was supposed to teach: the fact that they mastered the skills does not mean they will use those skills on the job: the fact that they do the job the way you taught them to does not guarantee that the business will benefit as a result. There must be particular learning objectives, job-related behaviour changes in mind, and tied to an identifiable business goal and plans to evaluate the course at all four levels to see if anything happened.