This is my take on how to tell when that brisket's done.

First, the goal: you want to be able to cut pencil-width slices across
the grain which hold together unless you take one end in each hand and
give it a gentle tug - then it should pull apart. You don't want it to
tend to shred as you cut it - by the time it gets in that condition, the
flat will be dryer than it needed to be in order to get tender. That's
overcooked. And of course you don't want it to be tough, which most
often would result from under-cooking.

Going from least to most reliable, here are some methods for estimating
doneness.

1. Time. Guess 1.5 hours per pound if you keep the temp between 225 and
250, but allow a margin of error of 1/2 hour per pound either way - most
often, in my experience, at these temps, if 1.5/lb wasn't right, you'll
need more, not less time.

2. "Fork tender" = "stick a fork in it and if it slides in easily it's
done." First of all, this should be done in the flat, since the point
will generally seem tender before the flat does. Second, I don't put
much stock in this test unless you use the same fork every time, and if
the tines are as sharp as they are on one fork I have, there's not much
difference in feel between poking shoe leather and filet mignon.

3. "Internal temp" - this, too, should be done in the flat, which is the
crankier of the two main muscles at issue. This is a little like cooking
by time - you can guess that you'll be done somewhere between 185 and
195, but one of my best briskets (which was choice grade, maybe it made
a difference) came off at 178 in the flat and 182 in the point.

4. "Fork tender" = "stick a fork in it (again, use the flat) and twist,
when it starts to feel easy to twist, it's ready." This is much more
reliable than the simple "stick a fork in it" test, but of course you
don't want to do a whole lot of poking and twisting to that brisket in
which you've invested so much tender loving care. You will feel the
noticeable difference this way, though, as the brisket "gives it up" and
goes tender on you - once you get the feel of it, it's an unmistakable
change.

5. "Wabba Wabba" = "poke the thing with your finger, and if it reacts
like refrigerated jell-o, it's done." This is not my idea, but was
published to a list or two by a fellow who may be the WSM brisket guru,
and once you get the hang of it, it's an amazingly good indication of
when your brisket's just gone tender but is still moist and has enough
coherence that it won't want to start shredding on you when you slice
it. Unlike the other tests, this one is probably best done at the middle
of the point; some flats are just never gonna bounce.

In practice, I'll keep a meat thermometer positioned about midway up the
flat, and I'll do the wabba and fork-twist tests. Once the temp hits 175
I watch it real close and when I get jelly-belly poking reaction and a
suddenly easier fork twist, I waste no time getting that thing off the
heat. This is mainly to catch the flat at the best stage. If only a few
of us are eating, I'll separate the two and put the point back in the
smoker for another hour or so - it's got enough collagen and fat cap to
keep it moist, and often benefits from the extra time in the heat.(This
is true even though it will often be the case that, when you first take
the brisket off the heat, the internal temp will be higher in the point,
than in the flat).