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.

[Marc Winter]

A few suggestions: 

If you haven't already, introduce yourself to the neighbors of the property and give them a card/contact info.  Let them know you are concerned about being a good landlord and care about the neighborhood.  Ask them to let you know if there is something out of the ordinary.

Try to find the 'mayor' of the block and talk them up.

Speak to the mail carrier, ask if there are any recent additional residents getting mail.

And definitely post a notice of lease violation--assuming your lease has a 'no additional residents' clause.