The first quality check that must be done before using any single subject data in a group analysis is checking the QA report produced automatically by BIAC (or you can run manually using the BIAC XCEDE TOOLS. Instructions are detailed at Quality Analysis.
It is important to visually check data before processing, as well as after preprocessing, and then again to look at the mask image for each subject. Since the human time to do all of this would be ridiculous, the Cluster Pipeline Tutorial includes examples of how to checking registration and printing a PDF for viewing, to print out a results report for each subject and convert the report to .pdf, use Art and ARTRepair to find motion outliers, and Coverage Checking in preparation for a group analysis.
As detailed in the SPM Cluster Processing Pipeline and above, the script produces a .ps file that can be viewed in SPM or converted to a PDF and viewed in acrobat. This file includes 12 randomly chosen .swu images for each functional run. Each of these images should look like a properly sized and shape brain. Any ghostly or warped images indicates that there were problems with registering the functional to the anatomical. In this case, an AC-PC realign will be required see spm cluster processing for details.