The first thing I notice that's a teltale about a girl is her facial expression, her hair style & upkeep, and do her clothes fit and match her body type/size. This is done in nanoseconds. lol! A perfect 10 = totally cool girl!
Haven't you seen "A Bronx Tale"? lol, The Door Test! Lock the doors (lol, the funny thing is, this took place in a convertible car) and when you unlock her door to let her in, you see if she reaches over to unlock your door. If she does, she's definitely a keeper. Most likely, she's a sweetheart.
In any respect - I'm thinking probably her nails! I mean think about it - if a girls nails are dirty - what do you momentarily assume? She is very dirty, she doesn't take care of herself and the list goes on about imperfections! But I'm also thinking - it depends what kind of nails she has on, can also cause how she is overall! There's a difference - really, really long nails make a girl look for some reason vulgar and uncanny and honestly in my opinion a woman with long nails just looks unattractive (no offense) but if she has normal size nails and you know with a nice french manicure - she looks sweet, innocent, nice! I at least think it's a girls nails unless it has something to do with clothes?!