I didn't see any distinctive markings, but I think the real give-away is that it was an effective multi-pronged attack, and there was no gloating transmission from some puffed up Imperial warlord.
Of course if it was Thrawn that means that he wanted Bo Katan chased off, not killed, otherwise he would have waited until she was on the ground to bomb the castle, then land troops. All he sent was bombers and interceptors and no troop transports; which means he knew she wasn't home, and that she was flying a Gauntlet.
As I said before though; the clearest evidence that it was Thrawn from a meta perspective is that we never saw where those fighters launched from. If it wasn't the Chimera, then there'd be no reason to obfuscate.
Of course if it was Thrawn that means that he wanted Bo Katan chased off, not killed, otherwise he would have waited until she was on the ground to bomb the castle, then land troops. All he sent was bombers and interceptors and no troop transports; which means he knew she wasn't home, and that she was flying a Gauntlet.
As I said before though; the clearest evidence that it was Thrawn from a meta perspective is that we never saw where those fighters launched from. If it wasn't the Chimera, then there'd be no reason to obfuscate.