Probably "mostrar de qué está hecho alguien". This means that someone has been put to a test where they must show their capabilities, for good or for bad.
- En el último partido el equipo tiene que mostrar de qué está hecho.
-> The last match must make the team show whether they deserve to win or not.
- ¿Quieres el aumento de sueldo? Pues ahora es el momento de que muestres de qué estás hecho.
-> The employee must prove they deserve the pay rise.
In sports, they also say "sacar la casta", which means to make an effort to show your actual capabilities.
- Parecía que el corredor iba a quedar en segundo lugar, pero al final sacó la casta y ganó la carrera.
-> It looked like the runner was going to end second place, but he made an effort and won the race.
- El equipo sacó la casta y logró empatar el marcador.
-> The team that was about to lose did their best and finished equal.
"Crecerse", might be another way to say something similar. This means you become better when you have to face an adversity.
- El equipo mexicano se creció ante el fuerte rival y ofreció un buen partido.
-> A weak team was expected to lose shamefully, but they rose to the occasion and they fought back.
- Juan se creció ante las críticas a su trabajo y desde hace dos meses es el mejor empleado.
-> Juan was a mediocre employee and when they told him so, he showed that he could do much better.
I can't think of any other ways to say this. Maybe someone else will find something more appropriate.
