The question is a bit general, so here is a general answer;

tcpdump support UDP mode. It will dump the packets as they traverse a network interface. If you know the port and address of the target you can see what traffic is being generated;

    # tcpdump udp port 53 and host xx.xxx.xxx.xxx
    tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
    listening on enp6s0f0, link-type EN10MB (Ethernet), capture size 262144 bytes
    00:08:31.213639 IP pc03.config.40641 > google-public-dns-a.google.com.domain: 5292+ A? tiles.services.mozilla.com. (44)
    00:08:31.213658 IP pc03.config.40641 > google-public-dns-a.google.com.domain: 189+ AAAA? tiles.services.mozilla.com. (44)
    00:08:31.214770 IP pc03.config.48792 > google-public-dns-a.google.com.domain: 33906+ PTR? 8.8.8.8.in-addr.arpa. (38)

If you have a local iptables, and packet logging configured, then you can inspect to see which rules are being hit;

    # iptables --list -v | egrep 'DROP|REJECT'
     6326  255K DROP       all  --  any    any     anywhere             anywhere             ctstate INVALID
    32200 7661K REJECT     all  --  any    any     anywhere             anywhere             reject-with icmp-host-prohibited
        0     0 REJECT     all  --  any    virbr0  anywhere             anywhere             reject-with icmp-port-unreachable
        0     0 REJECT     all  --  virbr0 any     anywhere             anywhere             reject-with icmp-port-unreachable
        0     0 DROP       all  --  any    any     anywhere             anywhere             ctstate INVALID
        0     0 REJECT     all  --  any    any     anywhere             anywhere             reject-with icmp-host-prohibited

To test that you can connect and send data between the client and server hosts. You can open a UDP port on the server like so;

    # nc -luk 172.28.128.39 667

and then send data to it from a client machine like this;

    $ nc -uv 172.28.128.39 667
    Ncat: Version 7.60 ( https://nmap.org/ncat )
    Ncat: Connected to 172.28.128.39:667.

    This is a test       <--- type this on the console
    This is a test
    This is a test

back in the server console, you should see this if the connection and packets were passed successfully;

    # nc -luk 172.28.128.39 667
    This is a test
    This is a test
    This is a test