We have a 1952 home with a fireplace in it (original to home). There is brick around the opening of the fireplace, then a wooden frame and then plaster about 8" past the wood frame and back about 8" to the wall. It goes up about 8" to the small wooden mantel and then appears to have plaster up to the ceiling from the mantel but that is only about 2.5" thick. Is there a way to tell if there is brick behind the plaster?
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).
You can also (apart from DA01's answer):
Drill a hole to see if a red 'powder' is what's inside.
Knock on it. Listen to how it resonates. It may require some masonery 'insight', but i bet it can help and You could distinguish if there's a open place (like between metal sheets) inside.
Combine few of these approaches to get an answer.