A usdm ITR will have b18c5 stamped on the left side of the block near where the tranny mounts. If its a gsr it will say b18c1.

If its jdm, both the ITR and GSR are stamped B18C. One way to differentiate is the intake manifold. A gsr has a much larger dual runner manifold, while an ITR has the same manifold as a b16.