How do I get the SQL statement generated by an X++ query?

For a Query object, call getSQLStatement() on the query itself.

Query query = new Query();
QueryBuildDataSource qbds;

qbds = query.addDataSource(tableNum(CustTable));
qbds.addRange(fieldNum(CustTable, AccountNum))
    .value(queryValue('US-001'));

info(query.getSQLStatement());

I use this when a dynamic query is not returning what I expect. Instead of guessing what the query framework built, look at the SQL it generated.

If you are debugging a normal X++ select instead of a Query object, use generateOnly and call getSQLStatement() on the table buffer. That is a different pattern and worth knowing too.