Detecting if gallery is ascending or descending

I have a pretty extensive set of galleries on my site, using WordPress’s built-in gallery functions.

Some of my galleries go in descending order, some go in ascending order. I have thumbnail prev/next links to cycle through the images, however my users have suggested that these be flipped as it makes more sense for prev link to be on the left when in ascending order, and vice versa for descending.

Seems fine to me. But I can’t work out how to do it!

All I need is something to slot in a PHP if/else statement that detects whether the gallery is set to ascending or descending, but because galleries are inputted with shortcode I can’t for the life of me work out where to get this.

Anyone have any ideas?