Customize checkout and cart page

Hello,

i'm trying to get a customization for marketpress plugin on my site.

I'm following tips listed on "theming_marketpress.txt" and using functions found on "template-functions.php".

I found a trouble while facing this page

Cart/Checkout Page
mp_cart.php

This file will replace two different pages....
store/shopping-cart/
store/shopping-cart/checkout/

And if i build up a custom cart on "mp_cart.php" how can i know if i am in one or the other page?

i mean...in the cart page i have to build the shopping cart...
in the checkout page i have the customer choose between payment options..are two different template.

I'm sorry but i can't figure out how to solve this customization!

thanks