How do you verify that a switch SVI (Switch Virtual Interface) has been correctly configured?

Use show ip interface brief and show ipv6 interface brief — they confirm the SVI's address and up/down state.

S1# show ip interface brief
Interface     IP-Address      OK? Method Status   Protocol
Vlan99        172.17.99.11    YES manual down     down

S1# show ipv6 interface brief
Vlan99          [down/down]
  FE80::C27B:BCFF:FEC4:A9C1
  2001:DB8:ACAD:99::1

What to look for:

• The VLAN (Virtual Local Area Network) interface should show your configured IP (Internet Protocol) address
• Status/Protocol will show "down/down" until the VLAN exists AND a device is connected to a port in that VLAN
• IPv6 shows both the link-local address (FE80::...) and the configured global unicast address

Tip: If the SVI shows "down/down" — check that: (1) the VLAN is created, (2) at least one port is assigned to that VLAN, and (3) that port has an active link.

Go deeper:

• Link-local address (Wikipedia) — the FE80:: address that always appears alongside the global address in show ipv6 interface brief.