To list all UDP connections, run

netstat -unap

Example

root@server48 [~]# netstat -unap
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
udp        0      0 198.27.105.231:53       0.0.0.0:*                           1718/named          
udp        0      0 198.27.105.230:53       0.0.0.0:*                           1718/named          
udp        0      0 198.27.105.229:53       0.0.0.0:*                           1718/named          
udp        0      0 198.27.105.228:53       0.0.0.0:*                           1718/named          
udp        0      0 192.99.18.195:53        0.0.0.0:*                           1718/named          
udp        0      0 127.0.0.1:53            0.0.0.0:*                           1718/named          
udp        0      0 192.99.18.195:38731     8.8.8.8:53              ESTABLISHED 16227/exim          
udp        0      0 192.99.18.195:45714     8.8.8.8:53              ESTABLISHED -                   
udp     4032      0 192.99.18.195:51742     8.8.8.8:53              ESTABLISHED -                   
udp        0      0 192.99.18.195:53170     213.186.33.99:53        ESTABLISHED 16227/exim          
root@server48 [~]# 

Scan All UDP Ports

To scan all UDP ports using nmap, use

sudo nmap -sU -v TARGET-SERVER-IP