How will you check whether automated build is successful and call a custom task on successful build?

You can do that by providing the following config entry in the MS Build or TFS Build config file,
<GetBuildProperties TeamFoundationServerUrl="$(TeamFoundationServerUrl)"
BuildUri="$(BuildUri)"
Condition=" '$(IsDesktopBuild)' != 'true' ">
<Output TaskParameter="CompilationSuccess" PropertyName="LocalCompilationSuccess" />
</GetBuildProperties>




<Exec Condition="'$(LocalCompilationSuccess)'=='true'" Command="do_the_voodoo"/>

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