Select orphaned rows (in a generic way)
Quoting the manual:

A foreign key must reference columns that either are a primary key or
form a unique constraint.

So that's not necessarily limited to the PK. But if we start with pg_constraint, we get all FK constraints pointing to the target table automatically. No need to provide any key columns - except if you want to limit to certain FKs.
Using the object identifiers type regclass and table aliases, we can keep the function short and the result safe and unambiguous:
Basic query
SELECT format(E'SELECT * FROM %s t\nWHERE  NOT EXISTS (SELECT FROM %s'
            , c.confrelid::regclass
            , string_agg(format('%s WHERE %s = %s)', c.conrelid::regclass, src.cols, tgt.cols)
                       , E'\nAND    NOT EXISTS (SELECT FROM '))
FROM   pg_constraint c
     , cardinality(c.conkey) AS col_ct
     , LATERAL (
   SELECT concat(CASE WHEN col_ct &gt; 1 THEN '(' END
               , string_agg(quote_ident(attname), ', ' ORDER BY fld.ord) -- original order
               , CASE WHEN col_ct &gt; 1 THEN ')' END) AS cols
   FROM   unnest(c.conkey) WITH ORDINALITY fld(attnum, ord)             -- possibly n cols
   JOIN   pg_catalog.pg_attribute a ON (a.attrelid, a.attnum) = (c.conrelid, fld.attnum)
   ) src
     , LATERAL (
   SELECT concat(CASE WHEN col_ct &gt; 1 THEN '(' END     -- parentheses for multiple columns
               , string_agg('t.' || quote_ident(attname), ', t.' ORDER BY fld.ord)
               , CASE WHEN col_ct &gt; 1 THEN ')' END) AS cols
   FROM   unnest(c.confkey) WITH ORDINALITY fld(attnum, ord)
   JOIN   pg_catalog.pg_attribute a ON (a.attrelid, a.attnum) = (c.confrelid, fld.attnum)
   ) tgt
WHERE  c.confrelid = 'my_table'::regclass -- target table name, optionally schema-qualified
AND    c.contype = 'f'  -- FK constraints
GROUP  BY c.confrelid;

Produces a query of the form:
SELECT * FROM my_table t
WHERE  NOT EXISTS (SELECT FROM schema1.tbl1 WHERE col1 = t.id)
AND    NOT EXISTS (SELECT FROM &quot;tB-l2&quot; WHERE (&quot;COL2&quot;, col3) = (t.col4, t.col5));

Which returns all rows that are not currently referenced by any FK constraint.
If cardinality(c.conkey) &gt; 1 then it's safe to also assume cardinality(c.confkey) &gt; 1. So only count once to decide whether to add parentheses.
Full automation
To make this work for any input table dynamically, create a polymorphic function taking a row value of the table:
CREATE OR REPLACE FUNCTION f_orphans(_tbl anyelement)
  RETURNS SETOF anyelement AS
$func$
BEGIN

RETURN QUERY EXECUTE (  -- exactly the query from above
SELECT format(E'SELECT * FROM %s t\nWHERE  NOT EXISTS (SELECT FROM %s'
            , c.confrelid::regclass
            , string_agg(format('%s WHERE %s = %s)', c.conrelid::regclass, src.cols, tgt.cols)
                       , E'\nAND    NOT EXISTS (SELECT FROM '))
FROM   pg_constraint c
     , cardinality(c.conkey) AS col_ct
     , LATERAL (
   SELECT concat(CASE WHEN col_ct &gt; 1 THEN '(' END
               , string_agg(quote_ident(attname), ', ' ORDER BY fld.ord)
               , CASE WHEN col_ct &gt; 1 THEN ')' END) AS cols
   FROM   unnest(c.conkey) WITH ORDINALITY fld(attnum, ord)
   JOIN   pg_catalog.pg_attribute a ON (a.attrelid, a.attnum) = (c.conrelid, fld.attnum)
   ) src
     , LATERAL (
   SELECT concat(CASE WHEN col_ct &gt; 1 THEN '(' END
               , string_agg('t.' || quote_ident(attname), ', t.' ORDER BY fld.ord)
               , CASE WHEN col_ct &gt; 1 THEN ')' END) AS cols
   FROM   unnest(c.confkey) WITH ORDINALITY fld(attnum, ord)
   JOIN   pg_catalog.pg_attribute a ON (a.attrelid, a.attnum) = (c.confrelid, fld.attnum)
   ) tgt
WHERE  c.confrelid = pg_typeof(_tbl)::text::regclass  -- input goes here!
AND    c.contype = 'f'
GROUP  BY c.confrelid
);

END
$func$  LANGUAGE plpgsql;

Call (important)!
SELECT * FROM f_orphans(NULL::my_table);

Or:
SELECT * FROM f_orphans(NULL::myschema.my_table);

Related:

PostgreSQL: Pass table as argument in function
Retrieving all PK and FK
SQL order by query results in any arbitrary (but reproducible) manner
How to preserve the original order of elements in an unnested array?