Is there a way to check emission sensors if the check engine light does not come on?

I have had to replace 2 emission-related parts (ie, CAM shaft sensor and throttle body [not the sensor itself]) within the last year and now the car is displaying signs that another sensor may be problematic (ie, slow acceleration, stuttering when brake is pressed, general sluggishness). These are the same syptoms that I had before the other sensors malfunctioned and had to be replaced. In both instances the car died just after the check engine light came on, leaving me stuck in the middle of the road. I'd like to prevent this from happening again since I drive the highway to work everyday but all the mechanics tell me they can't tell that anything is wrong until the check engine light comes on. The problems are not yet severe enough (apparently) for the mechanics to tell a difference in performance so all I've been told is to bring it back once the check engine light comes on. Which is fine unless I'm splattered across the highway 'cause my car died in the middle of the road.

Any insight would be much appreciated!