Checking if current entry exists in an entry custom field in a different entry
You can get the posts related to your "entry" (the currently shown project) with a
craft.entries element criteria model where you set the relatedTo parameter like so:

{% set relatedPosts = craft.entries.relatedTo({
    targetElement: entry
}) %}


Then check if this does find anything using Twig's length filter:

{% if relatedPosts|length %}
    &lt;aside&gt;
        {% for relatedPost in relatedPosts %}
            {{ relatedPost.title }}
        {% endfor %}
    &lt;/aside&gt;
{% endif %}