Options to Sub-meter Water in a Multi-family building

[Dan Bosak]

I found several old forums related to this but none that seem to answer mine and likely many other landlord's needs and I am hopeful that there is some newer technology that can provide a solution.  

I have several smaller multi-family properties with split utilities but a common water meter (4-7 units).  For one of my properties, the most recent bill equated to $283/unit/month in a building with average rent of $800/month. I suspect that it is 1 or 2 tenants in particular that have toilets running constantly and/or are just abusing the fact that they don't have to pay for water.  

Are there any options for sub-metering the water bills to individual apartments?

These are old buildings so the plumbing is probably not a direct run from the basement, so I would ideally need to measure water in at the point of use.  In most cases, they have separate hot water heaters, so I could just monitor how much hot water is used per apartment and take that % of the entire bill, this will not be exact so may cause other issues.

Any suggestions and ideally specific products for this or a company in South Jersey (Camden County) that does this would be great.

Thanks

Dan