It is common knowledge that the default SQL instance uses port 1433. However, when SQL server hosts multiple instances, or a custom port is being used, communication problems can arise. There are two easy ways to determine which ports are being used by a given SQL instance.
METHOD ONE: SQL Query/Stored procedure
Execute the following SQL Query (if one-off) or as a stored procedure (if executing multiple times)
The output should appear with the ports listed:
In this instance, port 61737 is being used.
METHOD TWO: SQL Configuration Manager
Open "SQL Configuration Manager"
Expand "SQL Server Network Configurations" and select "Protocols for "INSTANCE NAME"
In the right pane, right-click on "TCP/IP" and select "Properties".
Choose the "IP Addresses" tab, and the port will be shown at the bottom: