[hankaye]

FRANANDTOMMY, Howdy;

One way to check is to start with a empty black tank, hook up a non-drinking
water hose to the fixture in question and open the black tank (remember
I said it should be empty first), dump valve then turn the water on,
see if any water starts to come out of the black tank drain .. if so,
then you know.

hankaye

Ps. I'd put a bucket or something under the dump valve opening
to catch any leftovers from your last dump that might decide to follow
the law of gravity.