There's nothing assured in the direction when dealing with a crack.
However, what I have done successfully with low cracks is to cover the
area with silicone caulk on both inside and outside if possible. Let's
say the crack started at the lower left side of the windshield and had
progressed to a point 5" from that corner. You would block off a square
about 6" around that corner and the crack hopefully would turn to the
edge and not into the viewing area.
Obviously, if the crack is already in your viewing area you are
screwed. If it's down low you might try this technique. This method
has limited applicability, but it has saved me lots of money.
Tapping on glass is NEVER the way to go unless you are attempting to
remove the glass.