Multiple hall effect sensors in proximity
There are two ways of knowing this.  Mock it up and try it, having a variety of magnets of known strength around to try it with, or looking up the equations, grunting it out, figuring out the strength of the magnet you need, and ordering some sizes around that, then try it as above.

Most if not all of the equations you need will be on the Web pages of Hall Effect parts makers like Allegro.  You might also check out Wolfram Alpha.