How does Visual Studio /mstest identify test projects?

Say (100% hypothetically) that I've accidentally added a unit test project as project type "Class library" to a VS2010 solution. I've then added the assemblies needed to run it as a Unit Test project, but MSTest won't pick up on it when I hit "Run all tests in solution". What are the criterias here?

I had a couple of theories, which all have failed so far:

Something in the .testsettings file (no references to any assemblies here as far as I can see)
Something in the .SLN file (can't find anything)
Something in AssemblyInfo.cs (no, it's not)
Implict by referencing the (...)UnitTestFramwork.dll (Obv not)

Anyone?

Solution

In the project file, there's an XML element with the name ProjectTypeGuids, that holds a few GUIDs that denote test project. Example as follows.

<Project>
  <PropertyGroup>
    <ProjectTypeGuids>{3AC096D0-A1C2-E12C-1390-A8335801FDAB};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
  </PropertyGroup>
</Project>

Here's a list of known project type GUIDs for Visual Studio 2010: http://onlinecoder.blogspot.com/2009/09/visual-studio-projects-project-type.html

In the example above, it shows the project to be of type Test and Windows (C#).