Everyone has you covered mykyxd, especially with temps and probing. Just one point of clarification that SmokinVOLfan mentioned. Brisket is not steak. It is an extremely exercised muscle on the animal, full of tough fibers called collagen. That collagen has to melt for the brisket to become tender and juicy. It doesn't even start melting until 160-170F internal temp, and takes a long time to melt to the tender, juicy stage. An underdone brisket will be flavorful, but tough and dry tasting. An overcooked brisket will be tender, juicy, and fall apart (see PoukieBear's post above). It is just about impossible to overcook a brisket.
Also, you didn't mention what you are referring to as a "brisket." Around here, when folks say brisket, they generally mean a "packer," weighing 12-18 pounds consisting of the flat and the point. When new folks say brisket, they generally mean a 3-8 lb brisket flat with the point removed prior to being sold.
When smoking a packer, probe the flat for tenderness, not the point (thicker part of the cut, up to three inches), which is full of fat and will give you a false reading of tenderness. If cooking just the flat, usually only an inch to inch and a half thick, remember, it isn't a steak. An 8 lb flat can take 10 hours or more to become tender at 225-250F chamber temp, faster at higher temps.
If all this seems a bit overwhelming, you might want to wait on the brisket until you have more experience. Either way, good luck with the smoke and let us know what you decide.