Internal Memory

As in apps or pictures??

With apps (if you're using Apps2SD) you go to settings>application>manage applications> "On SD card"

For pictures etc. get Astro File Manager and with that you can browse the contents of your SD.

I am always running out of "internal" memory on my Eris.  You can run the "Manage applications" program. 

First run "Settings", then selection "Applications", then finally "Manage applications". 

It will take just a moment to build a list of all the applications on your phone and compute the amount of space each is taking up.  If you then click on any application it will show you space needed for application, data, cache.  You can also uninstall the app here or force close it.