You can tell the difference by wetting a corner of a white handkerchief and rubbing the carpet.
Rubbing the carpet with a dampened corner of a white handkerchief will always, without fail, allow you to spot the difference. 
mnli hypothesis: Rubbing the carpet with a dampened corner of a white handkerchief will always, without fail, allow you to spot the difference.  premise: You can tell the difference by wetting a corner of a white handkerchief and rubbing the carpet.