How to tell if your Iphone 5 is using Wi-Fi or 4G internet service?

I'm connected to Wi-Fi, but not sure if I'm still using my 4G service provided internet.

See the icons on the top left? See the bars then the provider (AT&T Verizon sprint) then next to that it will say LTE for 4g or three growing bars for Wi-Fi. You can also check in settings under Wi-Fi.

On the iPhone's status bar from left to right you will see the

signal strength > carrier name > internet connection* > time > any service you are using > battery (%)

*in the case of the internet connection if you are using Wi-Fi you would see 3 to 1 (depending on strength) waves.
If on a data connection you would see
-LTE on carriers that support it
-4G for HSPA+ (on AT&T)/3G for other carriers
-3G on carriers that support it
-E for EDGE
-o for GPRS

Whichever one you see at the time is the one that is being used.iPhones always prioritize to use the fastest option available. So if Wi-Fi is that then that is what it would use.