When purchasing a Shofar, is it necessary to look for a 'hechsher' to ensure that it was manufactured properly (e.g, no cracks), or can its suitability be determined just by visual examination?
Even if the horn comes from a kosher animal, the shofar could still be rendered non-kosher if non-kosher materials are added during the shaping and polishing process. According to this article (http://www.jdoorpost.com/2010/06/non-kosher-shofars-imported/), there were problems last year with shofars from China and Morocco.
Apparently, you can hide cracks. See this.
The issue is that cracks or holes can be filled, which invalidated the shofar, but is very hard to recognize.
Due to breeding, homones, etc., the horns are of a weaker and thinner-walled material. This make large shofars rarer, and the likelihood of tampered horns much more likely.