Given that about 10% of the faces have backflow, we should check what that is in terms of area percentage.

Think about what backflow is. Based upon that, you could use several techniques to find what are the velocities coming in at the exit. You could use contours at the exit. Or velocity vectors. Or there are several other ways. What I would use might be different that someone else. Modeling and result post-processing is an art. Do what you are comfortable with.

Regarding the area of interest: i am trying to calculate the pressure drop across a channel by substracting the pressure at inlet and outlet and i think my outlet pressure is computed at the exit so is it reason for creating the backflow?

You likely have the solver applies an average velocity of 0 [Pa] at the exit and a given velocity or mass-flow-rate at the inlet. This means at the outlet certain parts might be slightly higher or lower, but on average, zero. If the upstream values are lower, then the flow will tend to move upstream at the exit, causing backflow.

I would recommend that you set this complex model aside for a few days and work through some simple examples. If you do not understand the basics of the flow modeling, it will be very difficult to debug a complex model.