Without asking my respondents, is there a way for me to tell if they took the survey on a desktop or mobile device?

I would suggest adding a Meta Info Question to your survey. This is invisible to the user, but it will show you User Agent information. You can also graph this data in your reports like any other question.

Thank you, I didn't know this was even an option! I've been using Qualtric's for years and just never had a need until today. Thank you again.