How to catch illegal subletting?

[Bruce Chang]

To all you experienced landlords, short of installing cameras, how do you guys catch illegal subletting?  For instance, we've seen some unfamiliar faces in the past on our property, and they always claim that they are a friend of a tenant and just staying for a few days.  We're in a rent controlled city, and it's extremely difficult to evict tenants for subletting unless you have absolute 100% proof.