sexing rats

Question
Hello! I have 2 female rats that I adopted about 1 week ago. They are about 2 months old. I would like to get another female rat, as I have plenty of space, love, and food for them! I called a local pet store and they informed me they have several black hooded rats. Here's my questions:

How can I tell if they are female? I really don't want to have babies.
Also, if they are in a cage with males, how can I tell if they are pregnant? I know they can come home pregnant, and I would really like to avoid that situation.
Although I would of course provide a loving home for any rats I might acquire, I am really not prepared to have more than 3 or 4 at this time. Any advice would be helpful.
I do have one more question, and I know it might sound silly, but is there any way to prevent my rats from peeing on me? One of my girls loves to climb up on my shoulder and watch TV with me, but she always pees down my back. I would hate to have to stop her from sitting on me, because we both love it, but I'm really tired of having to jump up and shower constantly.  : -) Thanks for your help!

Rachel

Answer
Hi Rachel

Its pretty easy to see the male testicles at this age.

Check out this site to compare photos:
http://www.evergreenrattery.com/careguide/sexing.html

Also keep in mind rats can get pregnant as young as 5 weeks which you may already know.  What you can do is find a pet store that sells only rats kept as same sex. Petsmart is good for that.  
A female also has nipples and male rats do NOT.

You cannot tell if the female is pregnant until right before she is close to having the babies.  Gestation is right around 21 days and usually about 16 to 18 days gestation, they start to nest like crazy, preparing for the new pups. They may seem more aggressive and nervous or the opposite of their normal behavior in general. Their belly starts to take on the shape of a pear and the hair around their nipples starts to thin out so it can prepare for the babies to nurse.

Hopefully you dont end up with a pregnant rat since they can have up to 18 rats or more sometimes!

As for urinatijng on you....marinating is more like it.....LOL! It seems odd a female would do that to you. THis is more of a male trait!

I can tell you why they do this but cannot tell you how to stop her from doing it.  THis is just something rats do and here is why:

For starters, when a rat does this, they are marking you as their own. She may be the alpha of her colony and she is marking her turf, which is YOU!.
Rat urine can tell alot to another rat, too. It lets them know the sex and age of the rat and also even tells them if they are in heat or pregnant!  The main reason a rat leaves a urine trail is because in the wild, the rat will leave a urine trail to let himself know he has been in a certain area before. This keeps her from getting lost and this leads the way back to their nest.  Rats are pretty amazing.
If it helps any, rat urine is sterile till it hits the air. Although not the most pleasant thing you want running down your back or all over your clothing, the idea of urine in general tends to make everyone say YUCK! All I can tell you is to wear old clothes or put a dish towel over your shoulder while she sits there on you. It may be something she will outgrow. If she is new to you, she may be nervous and this is also a sign she may be a bit nervous yet.  
So long story short, if she doesnt out grow it, you will have to deal with it by either wearing old clothes and a towel over the shoulder as suggested or just not letting her on your shoulder, which you said you enjoy her there as does she.  Not much else to do to break her from it. Rats do not understand complex commands like how to have better bladder control. They are smart, but still too primitive to think anything else about it.

Hope this helps!