Its exactly as the above server posted about internal temperature. That is the only reliable method of testing for doneness, other than sticking your fork in and seeing if its "fork tender". Even a cheap $10 Taylor will do the trick. We just cooked one yesterday from Costco. It was 3.5 lbs in Hamilton Beach Crock Pot. We used WAY more than 1.5 cups of liquid recommended. In fact, with vegetables on the bottom of the cooker it required 3 cups of beef broth and 2 bottles of stout to cover the meat. The vegetables and brisket will float, so the liquid was to the top of the cooker. Although our cooker has a built in thermometer probe (with a hole in the lid), we dont often use that.
Now, while the USDA will say you need to reach an internal temperature of above 145 to 160, you must ignore that advice. You will have an incredibly tough, brick of meat. We cooked on Low. At about 4 hours the internal temp was 160 degrees and the meat is still a brick. At about 8 hours it was around 180 degrees. At about 10 hours it was at 195 degrees. We still had two hours to go until dinner, so we set the cooker to Warm, and just let it sit in the broth for two hours. During that time the internal temperature dropped from 195 back down to around 180. It does not make the meat tough letting it sit in a warm broth. We measured the temp by lifting the brisket out of the water and going in horizontally (assuming its a rectangular cut) to the center.
The resulting brisket was perfectly done. But even without a thermometer, its pretty obvious when the meat is ready. You can easily pull strips off the edges (i.e. its fork tender). The vegetables in the cooker are solely for flavor and aromatics. We never eat them. At the same time that prepare the cooker vegetables we essentially duplicate them, add new potatoes (cut in half), and place them in the fridge. About 30 mins before dinner toss the vegetables and potatoes with olive oil and cook in a 400 oven for 25 minutes. Too low an oven temp will lead to leathery vegetables. Too high will burn the bottoms. Both the vegetables (turnip, parsnip, carrot) and potatoes will come out perfect at the temp and time. If you add garlic cloves to the vegetables just know they will not carmelize in that amount of time.
For the cabbage, take two cups of the beer/broth in a large pot. Add the cabbage cut into about 6 wedges, cover and boil/steam until tender (happens quickly).