We can't, and we shouldn't. Voters as individuals cannot possibly control nor seek redress of their grievances from a government whose every action rewards or penalizes them as members of groups. Pressure groups are the only form that democracy can take in the age of the mixed economy and the welfare state. The problem is that no one can tell. But there is a deeper reason why corruption cannot be traced within our system, except in its grossest forms.
Corruption is the use of public resources for private ends.
From the individualist perspective of the Founders, a clear line can be drawn between public and private interests. The public interest is to maintain the institutions of law and defense that allow people to pursue their private ends. Today, under the influence of collectivism, the public interest consists in anything the public wants government to do. The line between public and private realms is not fixed over time, nor even determinate at a given time. The result is indeed, as reformers say, that a vague odor of corruption pervades the system.