You'll need salt water first. Freshwater doesn't foam in small skimmers. And by small I mean less than the size of a Buick. Then you need some protein of some sort to develop surface tension. If you want to watch your skimmer go crazy, you can put a capful of melefix or stresscoat in the water. Won't tell you much about the effectiveness of the skimmer however, any skimmer can foam melefix.
Mike