how to know which apps are running under multitask ?

What does it do?
What does it show?
What type of iPhone are you using?

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What does it do? - if it just shoves the screen up and show a bar across the bottom with nothing on it then nothing is really running.
What does it show? - if it shows "Favorites" or anything else, then your phone doesn't support multitasking.
What type of iPhone are you using? - Only 3Gs/4 do multitasking unless you've jailbroken 3G and enabled multitasking.

 

You probably have a 3G. No multitasking.