the easiest way to see them is to fly very low over friendly trenches or army depots. my latest Caudron G4 pilot has been having surprisingly good luck bombing them from altitudes of 5,000 to 7,000 ft. (even though I've been bombing from altitudes like this for a long time in flight sims there's still quite a bit of luck involved. especially since WOFF includes wind)
the other safe method of seeing infantry is to pick weapon view and watch the projectile blow them up at long range. (which is what I've done here)
if you open up the double-page screen shot in a new tab you can actually max the image out to full size. then you can get a look at all of the dead soldiers laying inside of the trench.