on Sales Orders, is there a field that I can use to indicate which method the sales order came into NS? for example - manually entered, or entered from a specific 3rd party app.

There is the source field, but not customizable. You'll typically only see things like CSV or WEBSERVICES there. We created a custom transaction body field for this so we knew which E-commerce store or other integration the order came from.

yeah did the same at our end. custom list and whatever method just selects it via web services etc

yes this the route we took in the end