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The default install includes netstat, which you can use to find out which ports are open, etc. For a more in depth portscanner the most popular is nmap.
netstat isn't really a port scanner. It shows network connections(tcp/udp), nic stats, routing tables, ect for your box. It can show what is listening on each port/ip but only for localhost.
Whether or not you are at risk depends on what ports are open. Anyway, netstat is much better for use on the local computer than nmap is. Run netstat -antuwp | egrep "(^[^t])|(^tcp.*LISTEN)" to find out open ports and what programs that are holding them open.
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