SETTINGS > LOCATION SERVICES
Whichever apps have purple arrows are the ones using it. Whichever ones have grey arrows are the ones that have recently used it.
What does it show in there?
Also, be aware that GPS and 'Location Services' are NOT the same thing. GPS is one of the things LS can use, but just because you see the LS arrow that does not mean the GPS was used. It could mean the phone was checking in with the cellular towers and got location info from there.