Assume CIC can and will find out everything. There are varying degrees to which things could cause you problems in your PR process, the the biggest thing that will cause you problems is lying to them. I understand needing to work to survive, but if you apply and tell them you've stopped all illegal work, and they find out that you're still working for cash under the table, you've officially committed misrepresentation and can be removed from the country, hit with a 5 year ban from entering Canada, and have fraud on your record with CIC forever which will seriously complicate any future attempts to even come visit Canada. Never lie to CIC.