Make sure that anyone that you hire to do PLUMBING is licensed in the State of Illinois or Chicago, either is acceptable with Chicago plumbers following more strict plumbing practices than state plumbers. Check that the company has a Plumbing Contractors license as well. A Plumbing Contractors license ensure the plumber is bonded and insured, otherwise they cannot renew their contractor license.
As a consumer it is within your right to card any person that comes to your house that claims to be a plumber. You can also ask to see a copy of their plumbing contractors license as well. For Drain Cleaning, the technicians do not have to be licensed, but the company does need a SEWER contractor license.
Make sure these technicians or companies that only advertise Sewer and Drain that they do not perform any plumbing repair or replacement as they are not licensed to do so. Get the price up front and read the terms and conditions on the proposal or invoice.