Show the audience that the deck you're holding is just a normal pack of cards. Show them all the cards to help them believe.  You can also shuffle the deck or allow audience members to shuffle the deck before you start the trick. "  They should be bewildered that you successfully guessed their card.

TL;DR in Vietnamese:
Một tay cầm bộ bài úp mặt xuống. Hãy hỏi khán giả "Lá nằm cuối có phải là lá át rô không?