Let me share my personal experience with you here. Honestly, there is no way or rather no single way of knowing that the crystal is working.
Think of it like an energy source which you can only sense and accordingly get guided. If you treat crystals like some robot or machine, trust me they will never work! But on the other hand, the more TRUST & LOVE you give them, they will manifest the same in your life.
While working with crystals, the most important thing to be kept in mind is PURE INTENTION and RECEPTIVITY. Just like you pray to GOD, similarly work with the crystals. Just as God never tells us a time when He will make things happen, same goes with crystals.
Also, follow the cleansing process ( which i have discussed in another blog ) once a week or two weeks for best results.
Om Sai Ram