As the brisket approaches the hour per pound point, it is time to take the temperature. You need to take the temperature in several places, aiming for a thick piece of meat. You should be able to feel the difference between the fat and the meat. By this point the fat is about as solid as warm butter. The meat on the other hand, should give some resistance as you insert the probe.
Look for an internal temperature above 175 degrees F. This is beyond well done for beef, but still too cool for a good brisket. Ultimately, you're looking for a temperature above 185 degrees F, so once it reaches 175, it's time to remove the brisket and wrap it in foil.