I have an old Fury X laying around and enough spare PC parts to build a mining rig. I’m just wondering if it’s worth it or if I should just sell it.
Whether or not it's "worth it" is largely up to you and your specific situation. If you've got the time, you may as well test out the card and see how your specific unit performs on various algorithms.
If your goal is profit, that depends on the actual performance of the GPU (and your speculation abilities, but we don't usually mess with that for profitability estimates at current conditions) and your power costs. (Plus how power-hungry your spare PC parts are.)
Alternatively, you can try to look up how the Fury X has performed for other users. *Usually* you can get pretty good estimates on how your unit would do.