Here are two things to look for: do the muscles along the BP's body feel firm and toned (good), or soft and mushy (bad)? Is the skin taught and smooth (good), or loose and wrinkly (bad). Keep in mind that while the snake is in shed the skin may get loose and wrinkly, and then tighten up after the shed. As long as the skin tightens up it is fine, if it stays loose and wrinkly there are probably some nutrition or hydration issues.

Finally, a disclaimer: a snake with firm muscles and tight skin is not guaranteed to be healthy, and a snake with soft muscles and loose skin is not always going to be unhealthy