The Mishnah states: "Ohr l'arbah asar bodkin es hachametz." (On the eve of the 14th day of the month, we check the chometz.) Rav Yehoshua of Belz zy"a asks why it says that we "check the chometz", rather than saying that we "check for chometz"?
He answers that "chometz" can be understood as a reference to a person's aveiros. When a person makes a chashbon hanefesh and realizes that he has sinned, he must try to determine what the source of the sin was so that he can completely uproot it. Therefore, he has to "check the chometz", meaning that he must investigate his sins to see where they came from and how they can be removed at the source.
His son, Rav Yissochor Dov of Belz zy"a, explained this with moshol of two merchants who traveled to a fair to sell their wares. They both were successful and sold all of their goods very quickly. They had more time than they expected, so they set off on foot towards home.
After several hours of walking in the hot sun, they got tired and wanted to stop to rest. They were worried about their money, and looked for a place to hide it while they napped. They looked around and saw that there was a herd of cows grazing nearby but no man was in sight. They then hung the money on a tree branch and went to sleep on the grass.
Unfortunately for the merchants, they failed to notice that the cows were not unaccompanied. They were being herded by a man who saw where they put the money. He waited until they were asleep and then quickly took it. He was now in a quandary as to what to do next. He knew that if he moved his flock, the cows would make noise and wake the sleeping men, who would realize that their money was gone and he must have taken it. He also did not want to run away on his own, leaving his cows behind. Finally, he came up with an idea. He took the money out of the sack it was in and filled it with cow dung so that the merchants wouldn't notice that it was empty. He then put it back on the tree branch and went back to grazing his flock.
His plan worked. The merchants eventually woke up and were happy to see that their moneybag was exactly where they had left it. However, they were shocked when they opened the bag and saw that it was full of manure instead of money. They couldn't understand how this could have happened. They wondered how the cows could have climbed the tree, taken their money, and filled their bag with dung. One of them said, "We must take the cows to the Rov for a din Torah!"
If they had any sense, they would have recognized that cows cannot climb trees and steal money, and there must be a cowherd with them who was responsible.
The nimshol is that every man comes to this world with a "sack", i.e., his heart. This container must be filled with "money", i.e., Torah and mitzvos. However, we sometimes fall asleep – meaning that we lose our focus – and the yeitzer hara comes along and steals all of our good intentions and deeds from our hearts, leaving worthless nonsense – meaning aveiros and sinful desires – in its place.
Once we wake up and realize that our hearts have been corrupted, we have to think about how this "dung" got into our hearts in order to remove it and get back the treasure it usurped. A wise person will recognize that the source is obviously the yeitzer hara, and he will chase after it and demand that it return what it stole from him, and, in the future, he will make sure not to "fall asleep" and allow this to happen again.
Accordingly, a person has to "check his chometz" by finding the source of his sins and eradicating it from his heart.