Dudes, assuming that you confirmed the presence of slag, what will you do on those welds that had been completed?
Will you suggest to gouge out the SAW and reweld it? if you want to raise an issue, then you must be prepared for possible solution
that is acceptable to Engineer in charge. Will that be the reason of having crack? why don't you just ask for PT or MT to check if there are similar cracks?
If you are acting as Inspector, then why not review the test plan to find out if there is a hold point for visual inspection on presence of slag prior to SAW process. If nothing is stated, then you may consult the Engineer to find out if this stage is critical.
If not critical, then stop thinking of the best NDT method