I'm creating an outbound message. I want it to show on non-mobile devices only. How would I do that? Is there an easy way or do I have to filter on various "OS" fields?

We use the various device fields for either IOS or Android as a filter point in conjunction with OS and we also use a URL/Path that we know differs for our mobile apps/services.

Another option, could be for your system to capture local device info (like Current OS/Device) and send that over as a custom attribute.

As there is no current device data point, it takes a bit of play to get it right for your system design.

Hope that helps.