Campaign contributions and voting positions
ismycongressmanbought.com
ismycongressmanbought.com
That said and taken into consideration, people that are outliers (like this guy: http://ismycongressmanbought.com/my_cong.php?con=Sodrel,%20M... and this guy: http://ismycongressmanbought.com/my_cong.php?con=Chocola,%20... should be viewed with a suspicious eye.
[1] Both happen to be Republicans. I was just viewing my home state (Indiana) where most politicians are Republicans. I am by no means trying to imply one party is better than the other. I am also a Republican.
More likely is that the candidate already supports a cause I care about, and I'm contributing to their campaign so they can get elected and continue supporting my cause.
If you want to see who's bought then look for people who voted one way on an issue and then switched their vote after getting contributions from the affected industry.
Which is unlikely to happen all that often. If I've got deep pockets and want to influence politics I'll focus on helping folks who already agree with me to get elected rather than trying to bribe those who don't. It's just a better bang for your buck.