ETM - using customFieldsMap to determine if data in custom field comes from a Shared Field

Hello. I am looking for information on how to determine in ETM whether a custom field value brought back from an event is a Shared Field. 

I have an Event mapper where I am able to bring back the customFieldsMap. One of the fields I happen to know is a Shared Field, but I want to code my mapper to verify that the value is coming from a shared field, but I can’t see any attribute which would identify the field as a Shared Field. 

This is what I get for the field:

"COST-CENTRE": {
                "imageId": 0,
                "customLookupValueId": 0,
                "assystRESTType": "customField",
                "assystRESTXMLType": "webCustomPropertyValueDto",
                "objectAvailable": true,
                "systemRecordFlag": false,
                "decimalValue": 0,
                "customFieldId": 79913,
                "version": 1,
                "entityInstanceId": 15400971,
                "customFieldShortCode": "COST-CENTRE",
                "singleSelectValue": {
                    "assystRESTType": "webCustomLookupValueDefinition",
                    "modifyDate": 1438686216000,
                    "modifyId": " ",
                    "definitionStateGroupId": -1,
                    "assystRESTXMLType": "webCustomLookupValueDefinitionDto",
                    "objectAvailable": true,
                    "systemRecordFlag": true,
                    "parentWebCustomLookupDefinitionId": 0,
                    "discontinued": false,
                    "version": 0,
                    "cacheable": true,
                    "dataLocale": "default",
                    "entityDefinitionType": 1,
                    "id": 0,
                    "shortCode": " "
                },
                "stringValue": "12345678",
                "cacheable": true,
                "dataLocale": "default",
                "entityDefinitionType": 1,
                "integerValue": 0,
                "id": 60940360,
                "customLookupValueShortCode": " ",
                "systemLookupValueId": 0,
                "customFieldType": 1
            },

Ideally, there would be an attribute which holds the ID of the shared field, which I can then use in an expression to verify that the data is what I want. 

Any help would be appreciated. 

Thanks, 

Duncan