You can use SQL Server Configuration Manager. Go to the node SQL Server Network Configuration. Find the server, select TCP/IP, right-click, select Properties and go to the IP Addresses and scroll to the bottom, and it will be one of TCP Dynamic Ports or TCP port.
You can also see it in the SQL Server errorlog. Around line 50 when SQL Server starts up, you will find a line like:
Server is listening on [ 'any' <ipv6> 42564] accept sockets 1.