Does iPhone automatically use wifi not my data?

It's connected to wifi 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 wifi, then it's not using any of your monthly data allowance. When you go out, you get Wifi Cafes or resteraunts and shops that offer free wifi for customers. If you're connected to one of those when you're out and about, then you're on wifi 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 wifi is on you won't use your data. But if its off your data will be used. The symbol for wifi is at the too of your screen on the left with is 3 curved lines, if you see those lines your wifi is on. But if you see 3G than your wifi is off and you are using data. Always make sure wifi is on before accessing the Internet.