find my iPhone is 'offline'
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.