BLDdir/bin/AtlasTest/<ARCHNAME>the .out file in that directory should be a complete text capture of the full testing log. More detailed output can be found in sibdirectories under here.
Once you have the error, you need to repeat it. You can try running the exact case, but sometimes that won't do it (for instance, you have a memory error that requires you to run many cases); you then need to find a small run that does demonstrate the error.
You should then apply the normal tricks (linking to F77 BLAS instead of sys blas, having the tester call the f77 blas twice, etc) to ensure the error really is in ATLAS, before tracking the error to its source.