How to test/read a stored procedure that returns a boolean?

I wrote this function but I'm not sure how to test it in PG Admin III
Query.

CREATE LANGUAGE 'plpgsql' HANDLER plpgsql_call_handler
LANCOMPILER 'PL/pgSQL';

CREATE TABLE handles (
handleID_pk serial PRIMARY KEY UNIQUE NOT NULL,
userID_fk integer UNIQUE NOT NULL,
handle text UNIQUE NOT NULL);

CREATE TABLE disallowedHandles (
handle text UNIQUE NOT NULL);

create or replace function IsUsedHandle(h text) returns boolean as $$
declare
begin
select COUNT(*) as num_matches from handles where handles.handle = h;
return num_matches > 0;
end;
$$ LANGUAGE plpgsql;

INSERT INTO handles (handle, userid_fk) VALUES ('blah', 0);

select * from IsUsedHandle('k');

Instead of true or false, it says

ERROR: query has no destination for result data
SQL state: 42601
Hint: If you want to discard the results of a SELECT, use PERFORM instead.
Context: PL/pgSQL function "isusedhandle" line 3 at SQL statement

If I wanted to call this query and get the boolean result in C++,
using PQgetvalue, how would I get this?

Thanks!