Hawzah News Agency – If someone doubts (i.e. has a shakk) whether it is the last day of Shaʿbān or the first day of Ramadan, it is not obligatory on him to fast on that day; and if he wants to fast on that day, he cannot do so with the intention of keeping the fast of Ramadan. However, if he makes the intention that if it is Ramadan then he is keeping the fast of Ramadan, and if it is not Ramadan then he is keeping a qaḍāʾ fast or another legitimate fast [including a recommended fast], the validity of the fast is not farfetched (baʿīd)(2) [i.e. the fast will be deemed valid]. In this situation, it is better that he fasts with the intention of keeping a qaḍāʾ fast or another legitimate fast, and in the event that afterwards it becomes known that it was the first day of Ramadan, it will be counted as the fast of Ramadan. Furthermore, if a person makes the intention of fasting in general [i.e. with the intention of attaining proximity to Allah without specifying any particulars about the fast] and afterwards it becomes known that it was Ramadan, it is also sufficient.

Source: https://www.sistani.org/english/book/48/