At the opening of the episode, *Vis à Vis*, Voyager encounters an alien vessel in distress. They scan it and detect one life sign. The entire bridge crew from then on refers to that pilot using the pronoun "he." [Examples][1]:

 - PARIS: We can't just leave him like this.
 - KIM: Nothing. Maybe he's ignoring us.
 - PARIS: Or maybe he's unconscious. We've got to do something.
 - PARIS: ... We'll tractor him in to kill his momentum, generate a warp field around his ship.
 - TUVOK: His flightpath is erratic. We'd be risking collision.

How did their sensors detect the alien's gender? Especially strange is that the alien at that time was actually an imposter from an unknown species impersonating a being from what was also a previously unknown species to Voyager.

  [1]: http://www.chakoteya.net/Voyager/417.htm