I have a feeling this is a dumb question, but here goes...

Flew the outbound, may have to cancel the return.
First want to know what I'd get back if I do so.

.com says:
Any residual value of a partially used, non-refundable ticket will be applied at the time of use.
Use your confirmation number to return to this canceled reservation in My trips and make changes for a future trip. Any residual value for the partially used non-refundable ticket will be applied when you change your itinerary. Change fees will apply.

...which, ok, I understand but I want to know what that residual value would be in order to decide whether it's economically viable to cancel, or if I should just take the trip as scheduled because I'm losing most of what I paid otherwise. So I called 1K, and the agent explained that the only way to find out is to cancel first.
Is that really the case or is there a trick to working out what I'd get back as a credit to be used on that same origin and destination?

Thanks!