SELECT count(*)
 FROM RDB$INDICES
 WHERE RDB$RELATION_NAME='TABLE_NAME'
 and RDB$INDEX_NAME = 'INDEX_NAME'

execute block
as
declare variable cnt integer;
begin
cnt=0;
 SELECT count(*)
 FROM RDB$INDICES
 WHERE RDB$RELATION_NAME='ITEMS'
 and RDB$INDEX_NAME = 'IND_ITEMS_MED_ID'
 into :cnt;
 if (:cnt<>0) then exit;
  CREATE INDEX IND_ITEMS_MED_ID ON ITEMS (MED_ID);
end;