Coded records from other sports tell us that there are more foul penalties called against visiting teams than against home teams. With a bit of effort we could check the validity of this finding when it comes to soccer, by type of tournament, field characteristics etc.

But the central issue will always be: do refs call more foul penalties against visiting teams because they are biased, do visiting teams foul more, or both? To answer this we would need computerized remaking of games, eliminating uniforms and ID's of any kind (faces included) and a panel of judges to evaluate possible foul situations.

Data, data. My horse for a data set.