I'm not shure what you're asking for...
Code:
if(my.skill40 != 0)
{
  // do stuff
}

?

If you want to keep track if a skill changed , you need to use another skill of the same entity. For example:
Code:
if(my.skill41 != my.skill40)
{
  // do stuff
  my.skill41 = my.skill40;
}

Extensive Multiplayer tutorial:
http://mesetts.com/index.php?page=201