I came across a text book example of a crack in an exhaust stack today at work:
The key to finding cracks is using lots of light from a good flashlight, and hitting it at the right angle. Here the light is directed straight at the crack, which is nearly invisible:
Now I move the light to the side, you can see a little of the crack but its still fairly hidden:
Once the light hits from above it becomes obvious where the crack is:
What can we learn from this example? The light needs to fall across the crack so that it can create a shadow. Similar to the early morning sun, the shadow will elongate as you move the light from a high angle to a low angle.
This is all about using the correct angle and making the light accent the surface texture.