A quick, naive question that hopefully the folks who have done same day trips far more than me can answer: is there a way to check in for a same day return flight prior to departure/within 24 hours?

I flew a round trip same day flight about a year or two ago, and the only way I was able to pull it off was to do it via CSR at the UC. Any other way to do it via the app, website, etc.?

Thanks.