find loop on cisco switch

Old school method for old/cheap switches with no CLI.

Front LEDs can provide indications like all on solid or call flashing in unison but not reliable across switch models.

WIth this method you need to know where all your trunk/uplinks are. Start by unplugging all trunks does the traffic stop ? If yes then traffic is probably coming from another switch start plugging back in trunks until the loop starts again. You should be able to find the switch and repeat the process there.

If the traffic does not stop then its coming from a server/user port or an unknown trunk. Start disconnecting each cable one by one until the traffic stops.

Cisco switch CLI method:

Can check ram and CPU usage is usually a good indicator or a loop.

Look at the 5 minute input rate packets/sec for the interfaces 

Looking for high values like 28458 packets/sec

show interfaces | in Gigabit|rate

You may have a mix of interfaces

show interfaces | i TwoGig|TenGig|TwentyGig|rate

show cdp neighbors | in "same-switch-name"

Wireshark

You can also take a Wireshark capture and look for high levels of arp/broadcast. Also look out for DHCP requests flying around again and again