Here is a very quick and sure way to differentiate this species from P. sericeus: note the large interocular space (dorsal gap between the eyes), and long temple (gap between the eye and where the head joins the thorax), which is larger than the diameter of the eye. (The temple is less than the diameter of the eye in sericeus). The difference is very clear when you compare the heads of the two species, and is easy to see in the guide images. And of course, impressifrons is generally a paler green, hence the common name Pale Green Weevil.
Regarding the mandible, please see my recent post in the guide page for P. impressifrons.