Oh that's easy.
(And forgive me because I'm not that familiar with eCommerce - I set it up for what I want and let it get on with doing that).
So items on sale get a little indicator in the corner of the product image, but other than that, there is nothing?
I would consider creating a notice for each sale. Text auto filled but editable by admin in the sale page. Then automatically displaying that (dismissible) notice when the sale is live. This will have the general effect of making the sale more visible to people who don't hit the home page or the products on sale.
(Delete the notice when the sale is deleted).
That's supplemental to my answer to you, but I would also use a different notice in the checkout page.
In the checkout page, if the user has selected some but not all of the products that qualify for the sale, I would display a notice notice to read something like
You have selected [1] out of [2] products which qualify for [Sale title]. Add [missing product title(s)] to your basket to automatically get a [50%] discount on your purchase today. Click here to add them now.
and I would link each of the missing products to the product page. And link the 'Click here...' text to automatically add those missing products to the basket and refresh the page.
You could put all sorts of stuff in the notice like the total they would save. Have the wording as a phrase or editable in the sale admin page.
It's somewhat of a significant change, but given all the effort you have put in to coding the sale feature, I think adding a notice would make each sale significantly more visible on the whole forum running eCommerce.
