How is Zakat to be paid on current bank accounts when it is unknown whether a hawl (one year) has passed on them due to the continuous use of the money?

The completion of a hawl (lunar year) is not a condition for the obligation of zakat on every single riyal. Rather, the condition is that a hawl passes on the money without it falling below the nisab (the equivalent of 595 grams of silver or 85 grams of gold).

When the hawl is completed and your wealth is not less than this value, you must pay zakat on all the amounts you possess at that time in all your bank accounts. This is obligatory according to Abu Hanifa, and permissible according to the majority of scholars for funds that have not completed their own hawl, provided they are not commercial profits. For example: if you owned 50,000 and added another 50,000 one day before the end of the hawl, then at the end of the hawl, you would pay zakat on 100,000, by giving 2.5% of it (2,500 riyals).

It is permissible to pay zakat for each account at its own hawl.