A molecular pattern trace was a method of determining the type and origin of a substance, such as a metal. This method would be used if other means of identification were not possible or yielded no meaningful results.
In 2368, USS Enterprise-D personnel, led by Geordi La Forge, used a molecular pattern trace to determine that debris recovered from a crashed Ferengi vessel had been a navigational deflector from a Vulcan vessel, the T'Pau. (TNG: "Unification I")