Look for switch ports with multiple MAC addresses off ports. There is a likely chance there is a hub or switch off those ports. If you can't see a CDP neighbor on such ports then go trace down the cables.

If there are ports with multiple MACs on them and you know these should only have 1 device then maybe you want to disable that port for time being. You could also use security features to limit ports to certain MAC addresses, etc but this is more admin overhead.

Theres also bpdu-guard which will disable a port with portfast configured that receives a BPDU on it (switches sends BPDU). In this case if a user plugs a switch in to their port and their port has portfast configured there port should be disable if their switch sends a BPDU out.

The best would be to have good control of the network and maintain an accurrate network diagram and have accurate port names/descriptions in the config of what that port is attached to.

You might be able to use a SNMP tool to probe the network and see what responds back but you need to be scanning the right management IPs on these other switches with right community strings.