Unusual plumbing problem

[Chris Modzeleski]

Hi, I've had a strange issue crop up recently, so I wanted to see if anyone had any advice.  I own a duplex in San Diego.  It's set up as two separate houses on a lot, and both are rented.  The lot slopes downward from the street, and both houses are below street level.  Because of this, the sewer line angles upward and there is a sewage ejector pump.

The pump burned out a couple months ago, and I had it replaced.  Now, my manager called today to say that it burned out again because there was some cloth caught in it.  The plumber thinks it was a sock.  Besides the idiocy of whoever flushed a sock down the toilet, I'm worried this may happen again.  If it was a single family house, it would be easy to bill the tenant for the repair, but since there are two houses, we don't really know who did it, and they both deny it.

Can anyone offer any ideas on how to catch the culprit and/or prevent this from occurring again?