wi fi - How can I tell that my phone uses the Wi-Fi connection for data traffic?

I just bought the Samsung Galaxy 2 and my monthly data plan allows me 250MB.

I have Wi-Fi at home with 100GB of data per month.

Obviously, I want to use my home Wi-Fi as much as possible on my phone, and I have no trouble connecting my phone to it.

My question is:
How do I know if my phone is accessing my home Wi-Fi and not my phone company's network when I access the internet on my phone.

I can see that my phone is connected to my home Wi-Fi but it is also connected to my phone company's network at the same time. Is there a way to see when I am using my home Wi-Fi and when I am using my phone company's network?

^{I am worried that if my phone connects to the phone company's network and not my home Wi-Fi, then I will end up with a huge bill. I just want to make sure I am using my home Wi-Fi when I am accessing the internet on my phone.}