How to do due diligence on rental rates for a turnkey investment?

[Josh Kelly]

What do you do to verify projected rental rates provided in a proforma by a turnkey provider?

I'm a new investor and am looking very seriously at a property with a turnkey company that comes highly recommended. The proforma numbers look fantastic (13% CoC returns after 10% for repairs and vacancy). They do in house property management and project that they'll have no trouble renting the property at $1595/month. In fact, they told me, they expect the property to rent within days of being finished and listed.

As part of my due diligence, I put the property into rentrange.com and it gave me $1254/mo rent with 85% confidence, but, to be fair, rentrange didn't take into account upgrades that are happening on the house. I don't know how they're algorithm works, and if it's really accurate. So next, I googled property management companies in the same city, found a random one, and called them asking if they could give me an estimate on how much they might be able to rent it for. The woman I talked to compared it to a few other properties she manages in the same area and estimated $1100 - $1200/month. 

I really want to trust this turnkey provider that they'll be able to get the $1595 rate for me. But I also want to verify it, to the best of my ability, before I go into the deal. 

Next steps for me will be to ask the provider to walk me through how they reached that rental assumption, and provide me with comps. But I'd also really like to verify with some kind of external sources, so I'm not just trusting their word for it.

How would you go about verifying the projected rental rate?