By UI:

Go to Site Actions >Site Settings(from your root site > Site Collections Admin > Usage Summary..
another method would be running the command line tool STSADM ENUMSITES (which is found here in MOSS07:

C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\BIN\ by default):

STSADM.exe -o enumsites -url htt:// YOURSITENAMEHERE > SiteList.txt

The results of the ENUMSITES in the SiteList.txt file will look like this:

Not very granular but quick and easy.