I guess the best way to really show what you can do is through word of mouth, testimonials. You can also document the pre and post tests and demonstrate great improvement but naturally anyone self reporting results can claim anything and you might be competing against truly bogus claims of another group. I can still see the value of using such results to sell a program. Another program where the coach knows not how to develop the athlete, will not take time to study nor plan out a program for that athlete might not be willing to test or even fabricate results-too lazy.
On the other hand they might rely on nebulous, unquantifiable numbers that were attained from testing procedures that maintained no degree of strictness or standardization and are only looking to be sure their athletes have impressive marks regardless of accuracy.
The better program is going to be detail oriented. It can obviously be difficult to promote a program when the “buyer” knows not what to look for, what makes a good program or a bad one. They do become enamored of the gadgets and toys.
Since health should be considered the most important outcome from training because no injured athlete can perform at peak levels, if a program repeatedly has athletes injured it’s bad. Unfortunately, some bad programs might not injure the athlete early on but eventually they will be hurt, not recover well on a consistent basis, just not perform well or all of these will occur. Only after some time wasted will the athlete and parent realize (maybe) they need to get out.
Other poor programs will show their true colors early on providing evidence that the program is unsound through injury.
It’s too bad that more parents and their kids don’t possess enough knowledge or experience in the subject matter to be able to properly evaluate program a from b.