The outlet of the compressor housing if it is already tapped, tap a hole in the inlet manifold, or anywhere in between (perhaps weld a socket in crossover pipe).
At the manifold will give a little higher reading than at the compressor, because of pressure drop - pressure drop is greater if you have an intercooler.
But anywhere mentioned above will indicate how the turbo is boosting. The main advantage in measuring boost at the manifold is that it will let you know if anything has happened inside the hoses between compressor and manifold.

