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Port scanning a target host for open UDP ports is very different from scanning for TCP ports in that TCP is connection oriented and UDP is connectionless; there is no tree-way handshake for UDP to synchronize sequence numbers.
When scanning UDP ports that are open and listening will return nothing whereas a closed port will return an ICMP Port Unreachable message. Because of the nature of UDP versus TCP, scanning for open UDP ports might take a lot longer than scanning for TCP ports. The result might also be less reliable and verifying ports stated as open by one tool, might be necessary by retrying the scan with other tools.
Most tools that do TCP port scanning will also do UDP port scanning.
When scanning UDP ports that are open and listening will return nothing whereas a closed port will return an ICMP Port Unreachable message. Because of the nature of UDP versus TCP, scanning for open UDP ports might take a lot longer than scanning for TCP ports. The result might also be less reliable and verifying ports stated as open by one tool, might be necessary by retrying the scan with other tools.
Most tools that do TCP port scanning will also do UDP port scanning.