1)Nohow. It's impossible. All information you have is that your solution have been hacked and this test does not contain in pretests.
2)You need to lock any of solved problems first. To do this click on the lock near problem's name. After that you are unable to resend your solution even if it is hacked. But you can view others' solutions and try to hack its. To do this simply double-click on someone's score in the score table.