Don't skip this step. Without it, meaningful tracking is impossible. First gain agreement on the problem and the value of solving it. Then outline how you propose to solve it. Establish a baseline measure of current performance, and clearly indicate how performance will be tracked and reported.
Proof is a figment.
Brilliant people assure me that it's impossible to isolate the impact of training. You can never tell whether some concurrent event has contaminated the results and negated their value as scientific evidence of training's impact, they say. Working with strong probabilities, make the case logically, linking learning to business results. Establish a causal link between a particular skill deficiency and a particular business outcome. If the manager buys into this logic and the way you will measure requests after training, that will be the proof you need.