Automated Test of Sorting by Column Feature

I am a new QA, and just started work last week. My senior left because of the working changes. Therefore, I have nobody to ask around me... and I took over his job. Hope you guys can help me here!

I am trying to write a automated test script to test the "sort by end date" feature. But sadly, I can not use selenium to test this feature from UI. Any thoughts of how to do this?

What I am thinking now, is that the sort on the front-end should be an "front-end" only feature? I do not know how to test it on back-end only. But my idea is that, I can call the API to create elements with different "end-date" then use LINQ to sort them. Then use the data from front-end and to make an assert statement to tell if the test is passed. But I am not sure how can I get the front-end data without testing from front-end....

If my description is confusing, please let me know. I will try to state it better.

Thanks!