I believe it's called your Ears Seriously, use them. If it sounds good to you, then it's good. You can also compare an original cd track against your mp3 encoded one. There's not really a better way to tell than that, IMO.
Encspot provides a very rough approximation(red, yellow, green), and it usually sides with bitrate, but it analyzes the encoding and guesses the encoder used, so if you want to have all LAME, for example, you can delete the other ones.