Edit entry, on the front end, with the entries field
There is a in operator to check if a variable is in a string or an array. Your entries field actually contains an object of type ElementQuery instead of an array of elements.

All you need to do is to check if the current entry is in the list of selected entries 

{% set selectedEntries = entry.planItems.ids() %}

{% set entries = craft.entries.section('exercises') %}
{% for entry in entries %}
    &lt;li&gt;
        &lt;input 
            {% if entry.id in selectedEntries %}checked{%endif%}
            type="checkbox" 
            name="fields[planItems][]" 
            value="{{ entry.id }}"&gt;{{ entry.title }}
    &lt;/li&gt;
{% endif %}