Agreed. If they doing cash jobs, there's no way to track it but if they are employed by a business who knowingly allows international students to go over 20hrs, the business can be fined if reported and such student be removed from Canada. Think about it this way, high chance nothing will happen but if something does happen you lose much more than a few extra dollars you earned while working illegaly.
Also, if you have to produce your paystubs in future to CIC for any reasons, they will find out what you were doing unless again it was strictly cash job. Also, CRA might be interested in cash workers.