detecting basic index page

i’d like to have a link back to my main index page on all my pages. i know most of the links (such as the comments and permalinks) just append values in the URL. so i know there’s a way of making a conditional that would echo a link if values were appended and would do nothing if no such values were found. any help? can i just check the value of ‘p’ if it’s set or is there something else i need?