In connected mode, you can use the LocalAddr() and RemoteAddr() methods of the connection object.
In disconnected (i.e. classical) mode, you get the address information with the datagram itself using one of the following methods:
func (c *UDPConn) ReadFrom(b byte) (int, Addr, error)
ReadFrom implements the PacketConn ReadFrom method.
func (c *UDPConn) ReadFromUDP(b byte) (n int, addr *UDPAddr, err error)
ReadFromUDP reads a UDP packet from c, copying the payload into b. It returns the number of bytes copied into b and the return address that was on the packet.
func (c *UDPConn) ReadMsgUDP(b, oob byte) (n, oobn, flags int, addr *UDPAddr, err error)
ReadMsgUDP reads a packet from c, copying the payload into b and the associated out-of-band data into oob. It returns the number of bytes copied into b, the number of bytes copied into oob, the flags that were set on the packet and the source address of the packet.
The address information is part of the return values in the signature of these methods.