Which is the method used to determine surface tension?

It may be easier to demonstrate surface tension to said 7th grader than explain it. Float a needle on water in a bowl. By way of explanation, the water molecules are "clinging onto each other" in the bowl, and the tiny forces with which they hang onto each other create surface tension. The vast numbers of water molecules hanging onto each other in the water that's in the bowl are acting like a very, very thin piece of fabric or cloth on the surface of the liquid.