As Chuck said, brisket is done when you can poke a toothpick or probe into the thickest part of the flat and it goes in and out with no resistance, like a knife through room temp butter. Start checking at about 185-190. For various reasons, some briskets are "done" at 190 while others might go up to 205+ before they are ready.
Once the brisket probes tender, pull it from the smoker and if wrapped, unwrap it and let it vent for about 20 mins. This is to halt the carry over cooking. Basically, you want them temp to peak and start to drop. If it will be awhile before serving, wrap the brisket back up and stick it in a cooler. If you'll be serving within 45 mins or so after it has vented, just seal it back up and leave it on the counter. It will be fine.
When it comes time to slice it, just cut what you need and be sure to go across the grain.
The biggest mistake most people make with brisket is that they don't have patience and they pull it too soon. Either they think that it HAS to be done by now (even though it isn't), or it's going past the deadline of when dinner is supposed to be ready. Monitor your cook. If it's not close to ready 1 1/2 hours before your preferred dinner time, figure out a way to crank the heat up. If this means pulling it out and sticking it in a 350 degree oven, then do so. You can safely hold a wrapped brisket in a cooler for 4 hours or more, so its no problem if you finish early.