There are several pieces of literature available from AWS that addresses the visual aspects of weld inspection and discontinuities. You might take a look at the two guides available from AWS. Guide to NDT of Welds (B1.10) and Guide to Visual Inspection of Welds (B1.11) would be a good starting point. Both guides are in the process of being revised.

You should be able to borrow them from your AWS Section library if you are an AWS member.

Best regards - Al