A weak signal from an 02 sensor is the primary reason that a computer controlled vehicle runs overly rich, which in turn can cause premature cat failure. Driving up a mountain however will create a significant load on the engine as well as the air becomes thinner -- in both cases the computer's fuel map will likely add fuel. Therefore, your situation (smell) may be normal. Yes, any good mechanic can pull and check an 02 sensor.

Usually, the computer recognizes a poor signal and will create a code that turns on the check engine/service light. The threshold for the error code, though, can vary. My sensor was operating poorly for a while before the check engine light came on - as a result, the car was running extremely rich under boost. But as they say...better to run rich all day than lean.