Debug/Release or whatever are just conventional values for the Configuration
property.
So, as long the project that includes/calls your .targets file adheres to the convention; you can check for debug mode as follows:
<OutDir>bin\Release\$(SomeOtherProperty)\</OutDir>
<OutDir Condition=" '$(Configuration)' == 'Debug' ">bin\Debug\$(SomeOtherProperty)\</OutDir>
or you could just use that variable directly:
<OutDir>bin\$(Configuration)\$(SomeOtherProperty)\</OutDir>