Bill - H2S or Hydrogen Sulfide smells like rotten eggs in lower doses but can lose the smell at higher rates. It is the second most lethal gas behind cyanide. Miners used canary's in cages to test for the gas. They make expensive monitors or test tubules. You could put a small amount in a can and let evaporate. The smell would get stronger as the water evaporated. I would be very surprised if you had any in your well water. Maybe you meant a test for sulfur?

Please no more rain for a month! :|