Can you check if the avatar is visible using the <window>.isCulled() function? This function takes in the name of the object and returns whether it is culled from the current viewing frustum. If the function returns 0, it's not culled so is visible. So maybe try this:
Code:
avatar = viz.addAvatar('AvatarYouAreUsing')
if(viz.MainWindow.isCulled(avatar) == 0):
// Avatar is visible