This is a very difficult question, one that has been of interest to political scientists for decades. It is basically asking how we tell who has power in a society.
On one level, this is simple. We look at disputes that arise in legislatures and we see which side wins. We look, for example, at whether the banks or the consumer advocates win when a vote is taken on regulating banks more closely. This was the approach taken by Robert Dahl in his classic book Who Governs?
But this might ignore other ways that power can be wielded. What if a consumer group wanted to propose a law but that law never even came to a vote? What if the banking lobby put so much pressure on legislators that they never even considered that law? That would show the bank lobby governing, but would not be noticed by Dahl's methodology.
Finally, what if some group is so powerful that it can prevent people from even perceiving a problem? Let's say that I think it is wrong for people to own property and I want a law passed banning private property. Hardly anyone thinks private property is a problem and so no one will even pay attention to me. Marxists argue that this shows that the rich have so much power that they have essentially brainwashed us into believing that property and inequality are good things. This is the ultimate power and it would show that the rich govern. (This was the approach taken by John Gaventa in his book Power and Powerlessness.)
Overall, then, it is very difficult to know who governs because it is difficult to actually detect when power is being wielded.