Might this be what you're after?

FUNCTION GETPRIMARYKEY(lcSQLTable)
*--Passed the name of a Firebird table, returns its primary key
*--Assumes giHandle contains previously-established connection handle
LOCAL lcSQL
TEXT TO lcSQL NOSHOW PRETEXT 15
SELECT RDB$FIELD_NAME AS COLUMN_NAME
FROM RDB$INDEX_SEGMENTS I
INNER JOIN RDB$RELATION_CONSTRAINTS C
ON C.RDB$INDEX_NAME=I.RDB$INDEX_NAME
WHERE RDB$CONSTRAINT_TYPE='PRIMARY KEY' AND
RDB$RELATION_NAME=?m.lcSQLTable
ENDTEXT
SQLEXEC(giHandle,lcSQL)
RETURN COLUMN_NAME