This research focuses on the processes of recognition of different professional knowledge acquired outside formal training systems. Beyond an environment that increasingly advocates lifelong learning, and considering that informal learning is not new, the recognition of this acquired knowledge through a diploma validation renews the question. After conducting exploratory field studies, an action training course was designed and implemented in an industrial company in the north of France with a group of operators. The first field results point to the discovery of the importance of a personal ability to know by oneself what one has learned informally, in a perspective of self-recognition, which can contribute to making VAE and on-the-job training more effective.