When some code is judged you can store it and if someone submit and you have same code stored than you can see if they are from same city or something like that. There are lot's of people who are solving Codeforces round so in some case for problem A code can be same. Also, you can create graph and if you see some cycle of length 4 or bigger than they need to get verdict "see you soon cheaters".
You think you can get red on Codeforces. That's right. There are two cases.
You have done 1000 tasks In this case you will get red easy by doing div2 and div1 contests. You don't have time to do 1000 tasks Just wait for new year and you'll become red! Easy!