Background check process will uncover it. If you're not upfront with it early in the process it will come up in background check and usually disqualify you.
Edit: background checks are standard practice for employees and contractors usually.
Why would contractors care, unless it's a high profile customer-facing thing like Uber?
I've done a lot of consulting and small contracts on the side and have never been aware of any background checking. I don't even think most have my SSN, just the info for my bank account. And I have a pretty common name. There's convicts even in my state with the same name I can find on search. Often there's not even a formal contract, just a handshake.
Also you can make money this way all over the world.
Edit: background checks are standard practice for employees and contractors usually.