Usually you can just tell by looking for marks on the fascia and small settings changes and new custom profiles. Also, look for any remaining files on the phone such as undeleted pictures or SMS text messages. Another thing to do is to check that all the default settings are present (these can be matched with another new phone), and that the default security codes are at default values. Granted, these can be changed, but it does not harm to check. Honestly in a professionally refurbished phone there is usually nothing wrong with it and it will be as NEW, but legally they cant label it as new.