If your page is missing meta[@name="viewport"] element within its DOM, then the following could be used to detect a mobile device:
@media only screen and (width: 980px), (hover: none) { … }
If you want to avoid false-positives with desktops that just magically have their viewport set to 980px like all the mobile browsers do, then a device-width test could also be added into the mix:
@media only screen and (max-device-width: 800px) and (width: 980px), (hover: none) { … }
Per the list at https://developer.mozilla.org/en-US/docs/Web/CSS/Media_Queries/Using_media_queries, the new hover property would appear to be the final new way to detect that you've got yourself a mobile device that doesn't really do proper hover; it's only been introduced in 2018 with Firefox 64 (2018), although it's been supported since 2016 with Android Chrome 50 (2016), or even since 2014 with Chrome 38 (2014):