Does iPhone automatically use Wi-Fi not my data?

It's connected to Wi-Fi so is it just using that and not part of my monthly data allowance? How can I check?

As long as it's connected to YOUR home Wi-Fi, then it's not using any of your monthly data allowance. When you go out, you get Wi-Fi Cafes or resteraunts and shops that offer free Wi-Fi for customers. If you're connected to one of those when you're out and about, then you're on Wi-Fi and not using any of your monthly data allowance. But when you're on them, it's really slow and you can't access a lot of things on your iPhone that need internet.

If you're on BT Open-Zone or BT-Hub, just don't disconnect from them and don't bother connecting. They don't work all at, and I think they secretly take away your allowance in my opinion, because it's So SLOW, and doesn't let me access Google Chrome or any games that need the internet, unless I want to pay. Then it'll remind me and let me know before entering Youtube for example, about data charges.
~Sophie:')

If your Wi-Fi is on you won't use your data. But if its off your data will be used. The symbol for Wi-Fi is at the too of your screen on the left with is 3 curved lines, if you see those lines your Wi-Fi is on. But if you see 3G than your Wi-Fi is off and you are using data. Always make sure Wi-Fi is on before accessing the Internet.