Excuse for field trips when students cannot attend to work. How can students proof they have shifts on Saturday?

Teaching university. I know there’s a separate sub for this, but posting is restricted there now.

We have two field trips every semester on Saturdays with mandatory attendance.

Some students cannot attend because they have to work on Saturdays.
How can I ask them to provide proof that they’re working?
In what way can they proof? A note from their manager? A selfie at work with a times tamp? Lol.

Edit: how can i close the thread?

I got some helpful answers already, and this is turning into criticism of a syllabus none of the people commenting here has ever read.