"Am I on WiFi?" (iPhone / iPad)

 Check your phone:

The more white the symbol is, the stronger the signal. 

If you have something else, like 4G, 3G, E, GPRS, where that symbol is, you're not on WiFi, you're using the mobile phone network for internet stuff. If you are abroad, this will likely cost you your firstborn.  You really need to switch roaming off.