how do I client script validation-saveRecord for Mark Shipped in Item Fulfillment?

You want to check if the status is Shipped or you want to check if it's changed to shipped?

what specifics you want to check?

no worries I guess I'll just throw an error in UE instead of CS

Be aware the user could potentially hit the Ship button or changed the drop down value. If using a UE you may have to rely on two checks, the trigger or the changed value.