With respect, depending on "customs" is wishful thinking, I'm afraid, Ted. No Jewish grave I've ever seen has had a cross on it.
If you really want to know if you have Jewish ancestors, you will need to do further research, particularly as very few Jews moved back to Germany after the Holocaust. You could also consider a DNA test, but I can't comment on them as no one in my family has done one. I'm sure others can help in that regard.