If the offer to play through is not offered, and you have a course marshal available, tell the marshal you'd like to play through. That's one of the reasons he, or she is there for in the first place...to keep pace of play. If not, and there is some place to go in front of the slower group, then ask the slow group yourself. Most of the time they will say go a head. In those rare instances when the slow group might say "no", then use you best judgement as to what you can get away with.