It seems to me that this should be able to be tested by checking whether the gravitational force between two objects of a given mass increases over time. If it

It seems to me that this should be able to be tested by checking whether the gravitational force between two objects of a given mass increases over time. If it does, then their mass has increased. If it doesn't, then their mass has not. (Of course, if the gravitational constant is correspondingly decreasing then this idea goes out the window.)