Ask the organizers what kinds of posters have been most successful in the past. If they have samples, look at those to determine the level of background knowledge that seems to work best. If not, ask the organizers where they've advertised the poster forum. If they expect to appeal to the entire community for a health fair, for example, you can assume little background knowledge. If they expect to draw in only math students from the highest-level courses offered in local high schools for a career fair, then assume viewers will have much more background knowledge.
