[rolly]

Do you know if there is any way to tell if the low quality is because of re-encoding or simply because it was a poor recording in the first place. As you know some CD's just sound bad and ripping them to mp3 just makes bad even worse.

I must agree here,having an original cd that sounds bad sure does happen,and nothing will ever fix it.
I don't know of any way to tell "why" the track has reached low quality without having the original to compare to,just that it has.

The calculator solaris brings up that RedSmurf posted is interesting.
I'm going to mess about with it for awhile.
But as mentioned,without the original file,I would suspect the figures would only be in the ballpark.
It would still be up to your ears to decide.

If someone ripped the original cd to .flac,which is lossless,
or to .wav which is also lossless,
the file sizes would be different.
Now,you can once again rip at 320kbps from:
1.the original cd
2.the .flac copy
3.the .wav copy.
and have 3 different file sizes,all of which could be considered very high quality.

To take a 128kbps rip from a very good quality original,or a lossless source,would surely have to sound a LOT better than a lossless rip from a original cd the sounds awful.
Again,the 128kbps rip from a very good quality original,would sound just as good,if not better than,a 320kbps file re-encoded from the same 128kbps file.
But not as good as 320 from a high quality source.

Using some apps,whether ripping to 128 or 320,there are input levels that can be manipulated.
A bad setting here even with 320 can make a lousy rip,and the 128 with better settings might sound better.

So the question of "why" the file is of low quality is one I don't think could ever be answered without having the original file present.