Let's say I have a task that I've know is going to take 12 hours to do. But when I schedule resources, I only scheduled 6 hours of resources. How could I show that the resources are under-scheduled? Perhaps a field that (or what calculations would I use on a custom field) that I can show [Work - Allocated Work]?
You can see this in the Resource Usage view. If you right click on the fields in the details column, you'll see a field "Remaining availability", which gives the information that you need.
Your work variance column should indicate the gap between your actual hours accrued plus forecast and the baseline number hours. So if you had 12 hours of work remaining per baseline but only forecast six hours to complete the work, you'll have a six-hour favorable WV in that column.