Limiting Excerpts to Appear Only On the Homepage

I’m new to WordPress and was just hired to work on a few bugs that require a very fast turnaround…. They want only excerpts to show on the main index page. Otherwise, they want full posts and they don’t mind that it could mean 20 full posts per page…. So:

1) What’s the best way to test for a home page or index page as opposed to any other page?
2) Where in the code or what file would I go to write some pHp to do this?
3) Is this a relatively easy thing to do?

i was unable to find a plug-in that would accomplish the same goal…although maybe I missed one out there….

Thanks!