If there was recent renovation on the property ask the owner if they paid all contractors and get copies of the lien waivers/paid invoices.

And in your defense the GC should have at the very least acknowledged receipt of the invoice and given a timetable for payment. 

Meaning you pay the bill but then you send an invoice to the tenant therefore making them ultimately responsible for the water that they use.

I had 30 days to get an accounting to the tenant and didn't have one of the invoices from my repair person yet, so I estimated it.  

@Sal Zizzo Request all the invoices, work orders and related documents on any transaction you are not happy with.  

My partner and I just finished our first rehab project and I've been getting generalized invoices.

Things like:- who directs the work- are there contracts in place for the work- how are the contractors paid (it is generally better to pay them on a job basis and from an invoice they present)- who provides the tools- do they have their own insurance- do they advertise- do they work for othersThose are just some of the tests that could be used. 

3) What sort of documentation or proof of work do you require (before and after photos, invoice, etc)?

The property is a triplex, converted out of an old house that's been taken care of very well (as far as I can see without a building inspector), so the water and gas are not split out into separate meters for the tenants (electrical is).In order to make this deal work, I'd love to figure out a way to invoice the tenants for those utilities.

You want to see applications, invoices, canceled checks, etc.