How to check if a link field uses  in template
Assuming your link field only allows 1 value.

{% if content.field_link.0['#url'].toString() is empty %} # or use "is not empty" if you're checking if there is a url. 
    # your code goes here
{% endif %}


If your field allows multiple link values then loop:

{% for link in content.field_link %}
  {% if link['#url'].toString() is empty %} # or use "is not empty" if you're checking if there is a url. 
    # your code goes here
  {% endif %}
{% endfor %}