There IS a warning...of sorts.....that lets you know you are getting low on fluid.........that is, if the rear window washer suddenly no longer sprays fluid on the back window, it means that it is time to top up the windshield washer bottle....
This is because the rear window washer fluid line is HIGHER in the reservoir bottle.....so once the level in the bottle drops below that rear window pick up line, you may still get fluid onto your front windshield , but it will soon be running out of fluid...
If you never use your rear window washer, well.........you are sol for a warning...........
;D ;D