A very useful table for viewing the status of all outbound HTTP / REST API requests is the Outbound HTTP Logs [sys_outbound_http_log] table.
You can navigate to System Logs > Outbound HTTP Requests
instanceName.service-now.com/sys_outbound_http_log_list.do

In these records, you can view very useful information such as URL hostname/endpoint, MID Server, response status, response time, source table, source record, etc.
You can also configure the log levels (basic, elevated) and depending on the level you will see more or less info.
Full docs here:
See more REST API tips below:
