Spectrals are easily the best method. It’ll give you a subjective idea, but it’s never going to be truly perfect with lossy encodes. If you’re looking at FLAC then you can tell pretty easily if something is a lossy transcode, but otherwise knowing if something is 192kbps vs 128kbps is going to be difficult. For really low bit rate reencoded as 320 that should be obvious on the spectrals, but it won’t tell you anything definitive beyond “well, it was a much worse bit rate previously”… which is useful, but only to a point.