Using the REST suiteql query endpoint or the records endpoints, is there a way to query NetSuite to get a list of custom fields?
You can use SuiteQL to list customfields.
For example to list entity custom fields:
{
"q": "SELECT * FROM CustomField where fieldtype = 'ENTITY'"
}
Use Setup -> Records Catalog to see the records you can query. It includes custom records and custom lists. ex. If you have a custom list called customlist_year then the records browser will include it and you can do a query like: SELECT * FROM customlist_year
with SuiteQL.
One caveat: SuiteQL is still in development so the fieldnames and results might change with a NetSuite release. Make sure things get tested with a Release Preview account so you are ready for any changes when they hit your production account.