Determining voter fraud can be quite difficult especially when things are close. First you need to secure all the ballots then count them again. Then you need to do things like take several thousand at random and investigate the voter to make sure they exist and are able to vote. Then you also need to check all the ballot for duplicates and compare them to what's on all the records.

People can also try to check to make sure there vote was registered and counted but it wont pick up all fraud.