one way to do this could be by first siding the sides by Cosine Rule (or Pythagoras) then applying sine rule in one of the traingles to find an accute angle. Find the alternate angle on another traingle, and if they turn out to be equal, then if two angles are alternately equal, then the lines must be parallel.

hope that helps

-sugarT

EDIT: easier would be just to do Cosine Rule to find the alternate angles.