WARNING: This page is about a different car, the 2008 Nissan Maxima. However, it is still accessible from the selected car via links, so may be relevant.
Use a reliable straightedge and feeler gauge to check the flatness of the exhaust manifold mating surfaces.
Limit :0.3mm (0.012 in)
Fig 1: Checking Flatness Of Exhaust Manifold Mating Surfaces