The category.regexExtractionProcessor log category should produce errors for some categories of failing to compile the TRANSFORMS regexes. You could turn up its verbosity to get success messages when building them as well. They do not have debug output after the first matching event comes through.
I've generally found it necessary to use a combination of pcretest, test instances/indexes, bogus fields, and careful tests to determine the cause of TRANSFORMS troubles.
Line breaking similarly will produce output saying what conf is loaded for category.LineBreakingProcessor, ie for a given conf/source/sourcetype, it will say what the truncation length is as well as the lookbehind (if these are set).
