Its often hard for the ref as he is 10M back but it seems that the linesmen have been deliberatlely instructed to ignore them as I've hardly seem one given all season by them.
PS I do understand the momentum rule before we start
Touch judges (not linesmen) can't flag for forward passes but do indicate to the referee when they believe that a pass was forward. These signals will often not be noticed by spectators. The referee chooses whether or not he takes any notice of such signals so, as in all cases, it is his decision to award one or not.
There always were forward passes but they occur (apparently) more often nowadays because the game is very much faster and they can be difficult to detect. In addition a flat pass is often a useful tool in opening up a defence so I'm sure that it's coached to some extent.
Referees say that they watch the angle of the hands/wrists as the clearest indicator of whether or not a pass was forward.