hi carleado ,
it depends on the model.by default the guest password for Cisco are ... 'BeMyGuest' for smart wifi routers and 'Guest' for E series router ans below. but if you already change the default do this...
>>access UI by typing 192.168.1.1 on the address bar to any browser and hit enter
>>router password is ' admin ' and if ask you for username type also 'admin ' and log in
>>look for wireless tab and click it
>>below there's a sub tab for guest access and click it
>>and you can see the password
>>for smart wifi routers once you access smart wifi tools you will see and guest window and password is there.
>>if you need detailed info email me...jjjm_1217@icloud.com