First one... there's no way to know.
Second one... cheating isn't detected by hacking into your computer :P... Engines play a different 'style' of chess that is very easy to detect with other engines. Statistical analysis would be done and you'd be banned in a week :).
You see, there is a rule in this site that said you can't cheat with chess programs during a game right?~ Well, I am not saying that I cheat but what do you do if senerios like the below happen?
First One:
Lets say, My friend is very good at chess and wins at a rate of 99%+ against rank 2000s. What if I ask him/her to help me play my games and tournies? Well, it is unlikely that you can find him or her or know that I cheat right?
Second One:
This may work in 1 computer but I want to make it tricky. I have 3 computers... 1 for gaming, 1 for working and 1 for chess. So one day I decide to use two computers for cheating. One computer has the super chess programs to help me win~ ( Enter the moves into the program to get my best moooove~ =) . Try to catch me :D~~
So... how do you know I cheated..>.<