A brisket is finished cooking when the internal temperature of the flat reaches between 203° and 206° F is great. When testing, the probe should easily go in without resistance. If it is still giving resistance, then let it keep cooking it for longer until it is. This will make a big difference in the tenderness. Use a remote thermometer so you can monitor the temperature wirelessly without having to open the smoker, then use an instant-read meat probe thermometer to spot-check and verify doneness in different parts of the brisket.