How do we know which graph api version we are using from the SDK

I think this can help you, because in this changelog it is written that in which SDK version which GRAPH API version was used.

• versions >= 5: https://github.com/facebook/facebook-android-sdk/blob/master/CHANGELOG.md
• versions <5: https://developers.facebook.com/docs/android/change-log-4x

For iOS, if you are not calling the method named + (void)setGraphAPIVersion:(NSString *)version anywhere in the project, you should be using the version defined with FBSDK_TARGET_PLATFORM_VERSION. You can find it in FBSDKCoreKit.h file as below:

Or:

You can also try to log [FBSDKSettings graphAPIVersion];