If you use a strong password with WPA2 encryption on your home WiFi network, and avoid using public WiFi networks, then it is very unlikely that your iPhone could be hacked. The hacker would need to actually have physical access to your phone in order to get control of it. But if you haven't been careful with security, then you can check the signs listed in the article previously mentioned. And always look for unexpected charges on your cell phone and credit card bills!
You can know or tell if your iPhone is being hacked or has been hacked by checking its operating system as done in a normal computer. You need to press the Sleep/Wake button and swipe the slider to access the Home menu screen. Check out http://www.ehow.com/how_5950700_tell-iphone-hacked.html for more information.