Any decent tire shop should have a set of calipers and be able to tell what the width is for you. Or if you're feeling crafty you could make a disposable caliper out of card board.
Their extremely basic. cut out 2 pieces resembling this, and just mark where the needle points and once you mark it measure the distance between the tips when the pointer is at the mark. subtract about 1/2 inch for aluminum wheels, and about 1/4 inch for steel wheels and that should be your bead lip width.
View attachment 296808