Since you've had five that didn't suit you,try one like Ribdog suggested.
Worse case,it will be fall apart tender,and then you will know.It should still be fine eating.
Best case,you will come out with good brisket,and you will then be on track to hone your favorite product.
Remember,what Ribdog suggested is a several step process,not a single snapshot in time.
Typically, brisket will be sliced about the width of a number two pencil.
If it tries to fall apart,slice a little thicker.
If too tough,slice more thinly.
In my experience,a 180* brisket,fresh off the cooker,sliced 1/4 inch thick,would take a couple of stout men to pull the slice in two.
But,that's just my experience.
Try again and take good notes.
Sometimes, it will still come down to the quality of the hunk of meat.