How can I tell if there is brick behind plaster on a fireplace?
Yes, you can cut a hole in the plaster and see if there is brick behind it. 

Perhaps an easier way is if this is on an outside wall and the brick no the outside goes all the way up. If so, odds are that it goes all the way up on the inside, as this would likely have been a full masonry chimney when it was built (as opposed to modern fireplace inserts which typically just have a metal insulated flue rather than a full chimney). 

Note that if your intention is to expose all that brick, you may be let down as often brick that was pre-determined to be covered is not the 'good' bricks and the mortar may not have been raked at all. Sometimes, they just plaster right on top of the brick as well, and that can be a long, tedious chore to remove (as plaster adheres wonderfully to brick).