So you need a lot of geese in each group. And that leads to one other point. In an experiment like this one, you will need to subject your results to statistical testing to demonstrate whether any differences between your control and experimental groups are significant or not. So mathematicians have developed a whole set of statistical tests to challenge the significance of different kinds of results. So you can see that this testing stuff has quite a few limitations and controlling factors involved.
Scientists are constantly on the lookout for flaws in their experimental design.
And for flaws in other scientists' as well, since every scientist is dependent upon the work done by everyone else in his or her field. This is another aspect of the self-correcting nature of science. There are a lot, but a couple of kinds merit a moment's attention. One indirect way to test a hypothesis like this is by using models. In the case of our goose, we could construct robot geese.