Filter tcpdump to display the packet containing the SNI header from the connecting client and make it human readable in the terminal output:
tcpdump -i <NETWORK_INTERFACE> -s 1500 '(tcp[((tcp[12:1] & 0xf0) >> 2)+5:1] = 0x01) and (tcp[((tcp[12:1] & 0xf0) >> 2):1] = 0x16)' -nnXSs0 -ttt