One way of telling the difference between a manufacturing flaw (either one of those little manufacturing cracks Glen describes, or a straw mark) is to look for evidence that it had happened while the glass was hot. This might include where the edges of the crack itself were deformed at all, crack was opened forming a valley. If the edges of the split are rounded (feel with a fingernail), then it's a straw mark. (Is that correct, everyone?)
Speaking of flaws, I have two pieces with what seem to at first sight to be cracks, which when viewed under magnification actually seem to be streaks of elongated bubbles and frit. Is this possible, or is it wishful thinking?