Many people are use brick to determine the age of a building. While there's is a big industry for making aged-looking bricks, you may need an expert to help you to discern between actual old bricks and copies.
Look for distress and chips. Though every brick is different, simply looking closely at a brick can tell you a lot about its age. The degree of distress is what will tell you roughly how old it is.
Learn the history of the area. In some areas, particularly next to a water source, learning some local history will help you to determine whether a brick is actually old, or just made to look the part.
Examine foundation bricks for water damage. Water damage from floods a hundred years ago would still be evident in bricks. If you know there was a flood that damaged some buildings in your town a long time ago, water damage could be the only clue you need.
Look for bricks that look charred or oxidized. Many cities underwent periodic major fires, and fire damage can be key at dating bricks from various eras.
Take a historical tour. If there are tours of your town's history, this is the best way to determine if a brick is legitimately antique. The tour guide will be able to tell you all about the town's history, which might including any building renovations that would have replaced the original antique bricks. Once you know which antique bricks are legitimate, you can check yours against them.
Look at surrounding buildings. More likely than not, the brick in a few of the surrounding buildings will look different than the rest. If it looks more distressed than the rest, the brick in that building is probably older than the buildings around it. If it looks newer, the brick is probably newer.
Check cornerstones. A building's cornerstone will tell you what year it was built, and if it retains the original cornerstone, chances are that the brick on the building is original. You will have to check against other sources to be sure, but the cornerstone is one more clue toward knowing for sure if the brick is original or just a good copy.