First thing I look at when dating brick is how large and how sharp the corners of the brick as a general rule of thumb. I notice the older the brick, the larger, cruder, and roundish cornered it has a tendency to be. Another thing is that older bricks have a tendency to be a little softer/chalkier.
I certainly don't let more modern looking brick stop me from detecting a site, you never know what was there before the structure.