Do you mean a D7100 vs something like a D3300, or two D7100s?
Yes, nearly every different version of generation of cameras will have a different sensor. But no, two of the same model should be exactly the same. If something is wrong, it's likely just a factory defect. Every D7100 should perform exactly the same under the same conditions.
Yes different bodies often have different sensors.For example, a D7100, will one show noise worse than another or are they all close enough to be the same?
How will you be able to tell the differences well enough to satisfy yourself that you got a "good one"?For example, a D7100, will one show noise worse than another or are they all close enough to be the same?