LET'S backtrack a few minutes before…
When Shen Ji Yun saw Bai Ze and Eclipse ambushing Su Yuqi and Luo Jin respectively, he knew that waiting for the battle to end and just watching everything from above was a waste of time. So, he decided to go directly and check himself which one of the two crystals was real.
He sent a message to both Su Yuqi and Luo Jin before flying straight towards one of the two crystals. The first one he picked was the one on his two o'clock direction. He didn't bother landing and just strike it with a sword slash. Purple lightning appeared and hit the crystal.
No, 'hit' would not be the right term since it literally passed through the crystal. Just by that, he immediately judged that the crystal was merely an illusion. Whether it was produced by an item or a skill, it didn't matter. What only mattered right now was the fact that it was not the real crystal of the enemy team.