This depends on many factors. Don't be misled, many sidewalk tearouts are hunted and little is found. However, every once and a awhile persistance pays off and you can make some great finds! Just be prepared to dig a lot of iron and trash. Ideally if you can hit an old part of town where the original grade exists below the concrete sidewalk that can be good. Sometimes there were no sidewalks at all and other times there were wooden plank sidewalks constructed in urban commercial areas. Concrete sidewalks did not really become common place until the early 1900's and that was only in the larger urban areas.