Suitescript - How to know the fields ID in sub record - Inventory transfer

I'm trying to create a restlet that can create a bin transfer. My code is almost complete, I can create line item fields also.

The only thing that I can't do is to fill up the inventory detail because I don't know the fields in Subrecords. How can inpect the field id in the sub records line items columns?

define([ 'N/record', 'N/search'], function(r, s) { function onSendRequst(context) {

    var rec = r.create({
        type: context.recordtype,
        isDynamic: true
    });

    rec.setValue( 'location', context.from_location_id );
    rec.setValue( 'transferlocation', context.to_location_id );
    rec.setValue( 'memo', context.memo );     

    for( var i = 0; i < context.inventory.length; i++)
    {
        var inv = context.inventory;

        // Create sublist Record
        rec.selectNewLine({
            sublistId: 'inventory',
            line: i+1
        });

        rec.setCurrentSublistValue({
            sublistId: 'inventory',
            fieldId: 'item',
            value: inv[i].item_ndc
        });
        rec.setCurrentSublistValue({
            sublistId: 'inventory',
            fieldId: 'adjustqtyby',
            value: inv[i].qty
        });

        rec.commitLine({
            sublistId: 'inventory'
        });


        // **CREATE A SUB RECORD**

        var itemInventorySubrecord = r.getSublistSubrecord({
            sublistId: 'item',
            fieldId: 'inventorydetail',
            line: x
        });


    }
    var recordId = rec.save({
        enableSourcing: true,
        ignoreMandatoryFields: true
    });

    return recordId;
}

return {
    post: onSendRequst
}

});