Only the ones with a teudat kashrut/kashrut certificate on display are kosher!!!
I know. But as far as I know, they all have it. Whenever I see one when I'm in Israel, it has it. Many years ago that wasn't the case and it was hard to find kosher fast food outside of Jerusalem.
Not all the Burger Kings are kosher but there is a list of kosher ones on their website. I haven't seen a non-kosher one in person because you have to go out of your way to see it unless you frequent the zoo called Tel Aviv. The only reason I even leave the train station at Tel Aviv HaShalom is to go to the one in the Azrieli Center and I bypass the zoo by way of a direct escalator over the highway into the mall.