How often have you been in a situation where the training was absolutely wrong for the participant?
I don't mean wrong as in, person will never use these skills, or wrong as in, person should be out selling instead of being in a formal learning program, or wrong as in, this is a very basic program for a very senior person, but wrong as in if a person applies the skills, they will do much worse at their job?
This might be because the program was not fully tested, or the wrong skills for the person.
If the answer is no, that training has never been harmful, then is the corollary true: that training can't do much good? Is training just warm tea and tic tacs to make participants a little better?
I look for evidence that training is getting stronger. Paradoxically, the thing that will be most convincing is a story of a training program actually being disasterously harmful.