Answer by spinaljack

save a time stamp for the last click and then check the current click to see if it is less than a short amount of time. e.g.

var lastClick : float = 0;

if(GUI.Button(Rect(someRect),"double click button")){
  if(Time.time-lastClick<0.3){
     // do something
  }else{
     // do something else
  }
  lastClick = Time.time;
  print(lastClick);
}