find my iPhone is 'offline' I lost my iPhone after I checked it in find my phone it shows that the device is offline. 

I want to know it means that the phone is 100% switched off or find my iPhone was not kept on in my iPhone [because I don't remeber correctly it was on or not] 

Does find my iPhone works on International Mobile Equipment identity number or Subscriber Identity Module?
You can not find out if the phone is 100% turned off.  The issue is that the phone may not have any network connectivity which would cause it to show as off line in find my phone.

The phone needs to communicate with Apple's servers and let them know that it is on and the GPS provides the location information.  If a person were to keep the phone on, but turn on Airplane mode, then the phone will show as offline in find my phone.

Sorry about you losing your phone...hope this answers your question.