<IsTfsServerBuild Condition=" '$(IsTfsServerBuild)' == '' ">false</IsTfsServerBuild>
<IsTfsServerBuild Condition=" '$(BuildingInsideVisualStudio)' != 'true' AND '$(BuildUri)' != '' ">true</IsTfsServerBuild>

# Simulate Visual Studio build
. msbuild.exe Project.csproj /p:BuildingInsideVisualStudio=true [...]

# Custom property
. msbuild.exe Project.csproj /p:MyCustomProperty=true [...]