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To get object datatype should be used the internal Virtuoso/PL function DB.DBA.RDF_DATATYPE_OF_OBJ , visible in SPARQL as sql:RDF_DATATYPE_OF_OBJ.

Suppose the following scenario:

# Explicit typecast (insert)
SQL> sparql insert into <test_datatype> { <a> <string> "string 1"^^xsd:string . };
callret-0
VARCHAR
_______________________________________________________________________________

Insert into <test_datatype>, 1 (or less) triples -- done

1 Rows. -- 94 msec.

#Not explicit typecast (insert)
SQL> sparql insert into <test_datatype> { <a> <string> "string 2". };
callret-0
VARCHAR
_______________________________________________________________________________

Insert into <test_datatype>, 1 (or less) triples -- done

1 Rows. -- 16 msec.

SQL> SPARQL
SELECT ?o (iri(sql:RDF_DATATYPE_OF_OBJ(?o, 'untyped!')))
FROM <test_datatype> { <a> <string> ?o} ;
o                       callret-1
VARCHAR                 VARCHAR
_______________________________________________________________________________

string 1                http://www.w3.org/2001/XMLSchema#string
string 2                untyped!

2 Rows. -- 16 msec.
SQL>