Shopify add a tag to a product that's on sale

product-grid-item.liquid

{% comment %}

 Add a tag to a product that's on sale. Add this in your template for displaying a product in a grid, for me it's called product-grid-item.liquid
 
{% endcomment %}
  
  
{% assign on_sale = false %}
{% if product.compare_at_price > product.price %}
  {% assign on_sale = true %}
{% endif %}
  
{% comment %}

 Work this into where the product image is displayed. I inlined the styles, but you can move them to a class if you want
 
{% endcomment %}
  
<a href="{{ product.url | within: collection }}" style="position: relative">
    <img src="{{ product.featured_image.src | img_url: 'large_cropped' }}" alt="{{ product.featured_image.alt | escape }}">
    {% if on_sale == true %}
      <h4 style="position: absolute; top: 0px; right: 0; background: #B41106; padding: 6px; color: #fff;">Sale</h4>
    {% endif %}
    
  </a>