{% if post.show_banner %}
    <div class="banner">
        <p>This banner is visible because the ACF true/false field is checked.</p>
    </div>
{% else %}
    <div class="no-banner">
        <p>No banner — field is unchecked.</p>
    </div>
{% endif %}

<p>Status: {{ post.show_banner ? 'Enabled' : 'Disabled' }}</p>

{% if not var %}
    {# do something when var is empty, false, 0, or null #}
{% endif %}