Might not fit as plugin is more about player ownership, but just a suggestion as it's what I need the most for my use case: 

        [HookMethod("IsDLCItem")]
        public bool IsDLCItem(ItemDefinition itemDef)
        {
            return itemDef.steamItem != null || itemDef.steamDlc != null || _redirectedItemIdToBaseItem.ContainsKey(itemDef.itemid);
        }​

If you only need a simple check like that you can just also check the isRedirectOf field in the item definition

public bool IsDLCItem(ItemDefinition itemDefinition) => 
        itemDefinition.steamItem || itemDefinition.steamDlc || itemDefinition.isRedirectOf;​

Good catch, thanks!

Merged post

Ended up with this, if it helps anyone:

public bool IsDLCItem(ItemDefinition itemDefinition) =>
        itemDefinition.steamItem || (itemDefinition.steamDlc && !itemDefinition.steamDlc.bypassLicenseCheck) || itemDefinition.isRedirectOf;​