In NetSuite, there is a field called SFDC Id. By default, this field is displayed on the Custom tab of the customer record. This field stores the Salesforce ID of the account this customer is synced to. To merge a record in NetSuite, you must ensure that the SFDC Id field contains the value of the Salesforce account you want the merged record to point to.
If you do not know the value of the SFDC Id field and need to look it up in Salesforce, it is very important to note that the IDs in Salesforce (as displayed in the browser) are not the same as the IDs needed by the integration. To work around this, Celigo has built a separate custom package that can be deployed to your Salesforce account to display the correct Salesforce API IDs. The package is available upon request and for a small setup fee. You can also use the Salesforce data loader to access the API based IDs.