antlr/doc/faq/actions-preds.md

Actions and semantic predicates

How do I test if an optional rule was matched?

For optional rule references such as the initialization clause in the following

decl : 'var' ID (EQUALS expr)? ;

testing to see if that clause was matched can be done using $EQUALS!=null or $expr.ctx!=null where $expr.ctx points to the context or parse tree created for that reference to rule expr.