Thongs and Skid Marks

I'll try to handle this delicately.  This is something that has confused and confounded me since I was a very young man walking on beaches in Florida and observing some fine (and some not so fine) female butt cheeks that were totally exposed due to very skimpy string bikinis.  I guess there are many variations in this "sports" wear--called g-strings, V-strings, T-strings, C-strings (yeah...try to imagine that), or thongs.  I think this all goes back to "thwong."  I know that thwong sounds kind of funny.  The first thing you think of is, "....wow--look at them butt cheeks..." and "thwooong".  But no, that's not what I mean.  Sure there may be a little bit of "thwooong" there, but what I really mean is that the name for this style of attire came from the old English word "thwong"--meaning thin, flexible, leather cord.  Uh, yeah.  From the back we have to assume that there is a thin--cord--in there, somewhere...we hope.  Anyway--I'm off the track here.  What confuses me now concerns the new fangled thong panties.  How do you know when you have skid marks in thong underwear?  Smell test?