Hello,
There currently isn't such a public API. There's a pending feature request to help discover such things:
But I'm currently not sure if opening up such API is a good idea - it's hard to keep the internal QueryPart APIs backwards compatible. Also, an aliased field is quite a different thing from the original field reference. For one, it can alias anything, including expressions based on several field references and other expressions, such as:
first_name || ' ' || last_name AS name
Of course, you can keep a reference to your unaliased field and fetch the qualified name from that:
Field<?> original = field( name( "MyTable", "myField" ) );
String tableName = original.getQualifiedName().qualifier().last();