As you now, at the contest your code runs at pretests, so it doesn't mean your solution is fully correct. If you think your solution is correct at the contest you can lock your solution at the dashboard, then go to your room and click solutions of other people in locked problem. If you think someones solution is not correct, click Hack button below his code then write test which his solution fails. That's all. Note that you can't resubmit after locking problem.

In Educational Div2 and Div3 rounds, you can see anyone's code after the contest and hack it within 12hrs after the contest ends. For hacking in those rounds, you have to open someone's solution and in the top of it there will be options to copy, hack, etc. Click on hack option. And then send the inputs in which the code is not giving the correct output. If your claim is correct, then successful hack otherwise not. But in case of Div2. rounds, there are some restrictions. You can hack the solution in your room only and that too during the contest only. You have to lock your solution first (if and only if pretests passed). For locking a solution you have to go to the Dashboard and there will be a lock symbol after the question name. Click on it and then confirm locking. After then go to your room and then you can see the solution of any of the participants in your room and you can hack it. But in case of unsuccessful hack you will lose 50 points. Also you can't submit again after locking your solution!! So be careful and confident while doing so!!