In order to catch voter fraud, you must first have a system designed to prevent it, and yet every election there are several isolated cases. In the US registering to vote requires very little, if any, proof that you are eligible to vote. Then when you vote, unless there is an ID law, you don't even have to prove that you are the person that you are casting the vote for. With absentee voting, it is even easier.

In Tennessee, the state I reside in, 2 legislative initiatives occurred about 12 years ago that make voter fraud extremely easy; motor voter, and no documentation driver's licensees (combined with our state medicaid program TennCare, it made us a very popular destination on charter bus lines from Texas). In essence you could obtain a license in Tennessee by simply asserting that you were over 18 and taking the test. The logic behind this is that there were illegal aliens living and driving here but they could not obtain insurance, so the state decided that it would help the citizens by allowing them (aliens) to get a license (and thus be able to get insurance) and the state would get the licensing fee revenue. A win for everyone, supposedly. A couple years later they passed motor voter which allowed you to register to vote when getting a license, good idea until you realize that a license no required no documentation. Now there are an untold number of people who are in this country illegally, who have a valid state issued licenses, and are registered voters, but they are not US citizens with the right to vote...

Still, there are examples of voter fraud every election in spite of how utterly moronic you have to be to get caught...