Praise be to Allah.
It is essential when giving zakaah to try hard to give it to one who is known or thought most likely to be entitled to it, and it is not permissible to be careless about that. We have explained the categories on which zakaah may be spent in the answer to question no. 46209.
But if you give it to a person and you do not think it is likely that he is entitled to zakaah, that is not valid and you have to give it again.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: The correct view is that zakaah is valid if it is given to one who you think is entitled to it after checking the matter in the case of uncertainty, and if you think that he is most likely entitled to it and you give it to him, that is valid. End quote from Sharh al-Kaafi
And Allaah knows best.