You should see a magic byte sequence of 30 82 (ASN.1 SEQUENCE). If you see all zeros, the device failed to sync the license.
With the introduction of and Single App Mode 2.0 , Apple is slowly phasing out the raw fmcert file in favor of encrypted license.plist blobs. However, the underlying cryptographic principle remains the same. The name changes, but the architecture persists. licensecert.fmcert