You can tell a lot about a girl just by looking at what she keeps in her medicine cabinet.  Is she high maintenace?  A slut?  Creepy?  Secretly a man?  Score a one-night stand and then snoop around behind the mirror to see if she's worth more sex (or if you're already fucked).  Take notes, kids.  This just might save your (sex) life.

Cosmetics:  Perhaps one of the most obvious signs that a girl is bad news is a cabinet full of makeup, hair gels, and other appearance-altering substances.  At the same time, it is safest to stay away from the girls with absolutely no cosmetic items above the sink, because that means that they don't care about their appearance all that much and will likely "let themselves go" once they have a steady partner.  Telling a personality based on cosmetic items can be a little tricky, so I'll make it up for you and you can go by that.  *disclaimer: if you get your ass kicked for making terrible assumptions, it is your fault for actually believing that a girl would tell you the truth about girls.*

Hair Products:
0-2:  Probably doesn't care what she looks like.  If she looks good now, it's entirely luck.
3-5:  The average girl.  This is the ideal amount, 4 being perfect, as long as they are acceptable hair products (see below for more information)
6-20:  Too many things, probably hiding something terrible and scary.  Asess your risk carefully.
20-800,000:  Run like hell and never look back.
Acceptable hair products: Hairspray, hairbrush, gel, hair ties, clips and minor decorations.
Bad signs:  Lice shampoo, dandruff treatment, wigs, combs with flowers, live animals.

Make-up:
0-1:  This is rather ambiguous.  She is either naturally pretty, or she's lying to you and hiding her makeup somewhere else.  The second option is more likely.  If she is hiding her makeup, that means that you won't know what she really looks like until you sneak up on her in the shower.  Prepare yourself for something scary, just in case.
2-10:  Ideal range.  Somewhere around 6 or 7 is the best.  Only a few colors of eyeshadow, one foundation, one mascara, and one blush are necessary.  You probably won't recognize anything such as lip liner, eye liner, or other pencil items, so I left them out of this summary.  (hint: Anything that you think is a colored pencil probably goes on her face.  Try not to think about it too much.)
11-50:  She has some gross deformity that she is hiding from you.  It might be an extra nostril.  You might want to back away slowly and never return.  If she has no deformity, she is extremely high maintenance and expensive.  You still don't want her.
50-*you can't count that high*:  She might be a man.  Spray her with a hose and see how much of her melts.  This girl could very well eat your soul if she chose to.  You're already in her house, so you're pretty much dead now.  Good job.
Things to look out for:  If there is anything that says "stage quality" or "movie makeup", it's meant to make you look like a completely different person.  Theater people use it to make young, pretty people into old, scary characters.  It can also work the other way.  You have been warned.

Other (nail polish, random glitter, body jewelry, things you think probably go into the above categories but you have no idea what it is):
0-1:  You're probably doing pretty good.
2-everything else: If it looks scary, the person who bought it is scary too.
Hint:  Eyelash curlers go into this categorie.  You don't know what that is.  It looks scary.  Only really high maintenance girls have them.  Scary girls have three.

Hygenic items:  Girls should keep themselves clean, so you should always check for things such as deoderant and perfume.  If it smells good, it's probably okay.  Girls should smell like flowers and happiness.  Too many perfumes and things are probably masking a bad smell, though.  Use your own judgement...actually, I take that back.  Your judgement sucks.  I know this because you're actually reading this.  Things that make a girl smell good should be limited to 5 things, at the most.  Other acceptable items are:
Facial cleansers, but no more than three.  Three is probably too many, but I'll allow a little wiggle room in case one is running low.
Toothbrush and toothpaste, one of each.  If there is more than one toothbrush, then there better be more than one person.
Hand soap is fine, but it makes more sense on the sink.  Refills may go in the cabinet.
Acne cream and/or wipes is acceptable only if there are two or less zit-related items in the cabinet.  We all need a little help sometimes, and at least she's taking care of it.
A good rule is that if it doesn't creep you out, it's fine.  Either that, or you're creepy too.  Just in case you can't tell if something creeps you out, here is a list of items that should send you running:
Facial hair bleach is a major warning sign.  It  warns you that she has a beard.
Tampons and pads do not go in the medicine cabinet.  If they are there, she's a little bit creepy.
An extreme amount of any kind of hygene product means that she is probably a germophobe and might disenfect your penis while you sleep.  Be aware of this and if you wake up smelling lemony fresh, sue her for assault.

Medications:  This is, after all, the purpose of the medicine cabinet.  The general rule is that if you see a medicine bottle, read the label.  The part that says "for the treatment of..." is all you need to know.  If you can't pronounce it, it's probably deadly.  This will also tell you how much of a whore she is, because you can count the number of STDs she's being treated for.  Here's another hint: lithium is a mood stabalizer, meaning she is being treated for some kind of mood disorder.  That's a bad thing.  Painkillers are normal, but an obscene amount of them could mean that she is addicted.  It's up to you to decide if that's a bad thing or not.  If her medicine cabinet is actually filled with medications, you should avoid her entirely.  She might use them to drug you and do perverse things to you without your knowledge.  You want those perverse things to be done while concious.  Only you can decide how much disease you will tolerate, so it's up to you to choose whether or not to stick around.

There are just a couple more things to remember.  If she has set the marble trap in her medicine cabinet, she is smarter than you and thus out of your league.  If she has a note in her cabinet telling you to get the hell out of her stuff, she is still smarter than you, and probably meaner, too.  If she has sexual items in her cabinet, take them and try to convince her to use them.  She's probably just your type.  Try to be sneaky, because girls hate it when their "personal space" is invaded, but you need some evidence to decide if you want to invade her other personal space.  Happy hunting, boys.