If an ingot has a bad bubble problem, it usually is visible on the outside. Occasional small bubbles are easy to work around, so even if they can't be seen they are not a problem. Cracks that mean an ingot is no good usually show up right away, and in descriptions of historical ingot testing that I've read, that's the first thing they checked - how they responded to the first few hammer blows.
The ingot pictured above started cracking (in an ominous fashion) right away, but I kept at it to see if I could coax some useable metal out of it - no such luck!
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