Actually, I don't really understand how to implement it using API check "method".  I thought a lot about this and the only case I've found is following (very bad approach):
<code>
    @Override
    public boolean check(final String login) throws IOException {
        try {
            this.request
                .uri().path(login).back()
                .fetch()
                .as(RestResponse.class)
                .assertStatus(HttpURLConnection.HTTP_NO_CONTENT);
        } catch (AssertionError e) {
            return false;
        }
        return true;
    }
<code>