There ARE of course ways to test your mp3s for missing frames, bad data, etc. I have a process that weeds out all the trash.
You can use Encspot's Sync Error column (right click the columns to customize and add it) to test for sync errors in mp3s after downloading them. I use a program called mp3test
http://home.arcor.de/shivi/MP3Test/ , and the program which is based (a commandline tool) called mp3check
http://sourceforge.net/projects/mp3check/ . mp3test will catch all of Encspot's errors, but will also catch truncated mp3s. This can be good or bad, as you can't set the program to skip these as mp3check does, and it really doesn't matter if your mp3s are truncated, as this can be fixed later. Also, MP3test used to be freeware, but is now shareware. It now supports vbr mp3s though.
My advice is to first use the commandline mp3check to check your files, it can output lists of problem mp3s depending on the settings you choose. I usually have a commandline set up for each test, as this allows me to find out which problems effect which mp3s. First of all though, I can also use mp3check to clean up my mp3s as follows.
1. I use the following commandline with mp3check to remove all tag info and cut all bad data from the beginning and end of all the mp3s. I have made sure that this commandline does not "fix" or resample mp3s with missing samples (resync errors). Often though, this does the trick for the majority of problems. My philosophy is that if your mp3 DOES have resync errors, download a fresh copy.
mp3check --recursive --cut-junk-start --cut-junk-end --cut-tag-end "Directory"
2. Next I use mp3test or Encspot to find mp3s that are still bad, meaning they contain resync errors. Those are the ones that I delete, because they are irrecoverable. Mp3test is nice to use here, because it will put the bad mp3s into a separate "bad" directory for you if you choose.
3. With what is left, the good mp3s, I run them through mp3trim to remove silence from the beginning and end of the mp3s. Any truncation that occured by removing bad data from the ends of the mp3s with mp3check "disapears" when I do this, because a new clean beginning and end for each mp3 is created. Make sure you run mp3trim AFTER you remove the bad mp3s found in the previous step, because mp3trim WILL repair the headers of bad mp3s, making them seem "error free", but leaving the pops and clicks from their resync errors. BTW, mp3trim is freeware, but has no batch mode and has a file size limitation. There is a shareware version of mp3trim, the PRO version, that IS batch with no filesize limit, but it costs a lot of money.
4. My next step is to normalize with mp3gain. After normalizing, I move any files that still clip after radio normalizing to a separate folder to remind me to seach for that mp3 again, hopefully finding a better (more amplified) recording. Usually, this is to no avail.
5. To fix file namings, I use a program called Mp3Renamer
http://www.dgdr.com/ that is no longer supported but is still availiable for download. This program auto capitalizes and fixes common errors. For unique mp3 names, i go in manually, or use a standard mass renamer program (there are many). Next, I use a program called TagScanner
http://xdev.narod.ru/tagscan_e.htm to read the filenames and create id3v1 and id3v2 tags and artist and album directory structure.
All of the above is not needed if you use EAC and your own cds, which is what i do for my own albums, but should be a fine solution for fixing downloaded music. Hope this helps.