How do you confirm what speed and duplex a switch port actually settled on?

Question

Accepted Answer

show interfaces <interface-id> — its first line gives the up/up state, and a few lines down the output names the duplex and speed the port is really running.
S1# show interfaces fastEthernet 0/18
FastEthernet0/18 is up, line protocol is up (connected)
  Hardware is Fast Ethernet, address is 0025.83e6.9092 (bia 0025.83e6.9092)
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, media type is 10/100BaseTX

The distinction worth internalising: show running-config tells you what you asked for, show interfaces tells you what the port got. On a port left at the default auto/auto the running-config says nothing about speed or duplex at all — that answer only comes into existence once autonegotiation has run, and it lives here.
What to read in the output:

is up, line protocol is up — physical carrier and Layer 2 keepalives are both healthy.
Full-duplex, 100Mb/s — the negotiated (or manually forced) result. This is the line that settles a duplex argument.
BW 100000 Kbit/sec — the interface's bandwidth value, which tracks the negotiated rate and feeds routing metrics on routed interfaces.
MTU 1500 bytes — the maximum payload, the number that makes an oversized frame a "giant".

Tip: When a link "works but feels slow", read this line before touching anything else. A gigabit-capable port reporting Half-duplex, 10Mb/s is a failed negotiation, and the fix is almost always at the far end rather than in the cable.
Go deeper:

Troubleshoot Catalyst Switches to NIC Compatibility Issues (Cisco) — Cisco's own walkthrough of how autonegotiation settles speed and duplex, and how mismatches show up.
 Ethernet over twisted pair (Wikipedia) — the 10/100/1000BASE-T rates and media types the port is choosing between in that output.