We can check if that specific method available in this version of sdk or not.

For example there is method AVCaptureAutoFocusRangeRestriction of class AVCaptureDevice which is available in iOS 7 but not in iOS 6. So to check we can do the following :

SEL sel = @selector(setAutoFocusRangeRestriction:);

if([device respondsToSelector:sel])
  {

     // calling function available in iOS 7
      NSInvocation *inv = [NSInvocation invocationWithMethodSignature:[device methodSignatureForSelector:sel]];
      [inv setSelector:sel];
      [inv setTarget:device];
      int AVCaptureAutoFocusRangeRestrictionFarValue = 2;
      [inv setArgument:&AVCaptureAutoFocusRangeRestrictionFarValue atIndex:2];
      [inv invoke];
  }
else
 {
    call function available in iOS 6
 }