How do I know?

So, I was listening to a program on the christian channel in the car on my way home today, the man was talking about how when the 10 commandments were issued taht it only said not to commit adultury and how the israelites thought it meant physically, well Jesus told them that it was not only that way, that if you lusted after someone with your heart or mind you were just as guilty as the person who did it physically. And that when we will be judeged on judgement day that God will look only at our hearts and that our thoughts reflect what's in our hearts, (well not only, but for all intents and purposes here, it will be) So my question would be if I have bad thoughts about someone whether lustful, murderous, or otherwise, does that mean my heart also turns bad?