It depends on the facility and the lighting.
Sometimes it's possible to see at least a silhouette of most of the audience. Other times it's only possible to make out the first few rows. And occasionally (especially if in a spot) it's difficult to see anyone in the house at all.
"You just can't win. Ever. Look at the bright side, at least you are not stuck in First Wives Club: The Musical. That would really suck. "
--Sueleen Gay