Well, here are two ways to check if a record is either a Standard or Custom record:
Check the Custom records listing
Customization > Lists, Records, & Fields > Record Types
Check the URL of the record
If the URL has custrecordentry.nl in the latter portions, then it is a Custom record.
i.e. https://system.netsuite.com/app/common/custom/custrecordentry.nl?rectype=1&id=1