>>>I currently rank on the 1st page for several of my website pages, but I am having a hard time determining what keywords played a role in getting me there. <<<
You already know the answer to this question. You say you currently rank on the 1st page for several of your web pages. You rank on page 1 when you search for what particular keyword or keyword phrases? The keyword you searched for when your site was returned on page 1 is the keyword (or phrase) that caused you to rank.
There are lots of factors that likely made your page rank for a particular term or phrase... Some of the big reasons (in what I consider order of importance)
1)Relevant sites/pages probably link to your page using that particular keyword or phase (or some variation of it)
2) Your <title> tag probably contains the keyword or phrase
3) You header tags <h1>, <h2>, etc. might contain the keyword or phrase
4) the keyword of phrase might have appeared multiple times throughout the content on your page.
5) Perhaps the URL for the page is a keyword rich URL (the keyword or phrase or some variation of it might appear in the URL).
These are but a few reasons why you might rank for a term. Typically it will rank because of a combination of several of these and more. But #1 alone CAN make you rank if you have enough incoming links from relevant pages with that particular keyword or phrase in the link text.
I have pages on my site that rank on page 1 of the SERPS for many terms that don't even appear on the page or in the title tag or in the header tags or in the URL... My page ranks on page one because so many sites link to my page with that particular term as the anchor text. But I have 1.4MM inbound links.
So links on pages at other relevant sites to your page with the keyword or phrase in the link text (and also around the link on the other site's page) carry a LOT of weight toward telling the SEs what YOUR page is about. I'd say this is the single most important factor. Title tags carry a LOT of weight as well. I'd rank title tags 2nd in importance... The header tags, content on your page, keyword rich URLs, etc. are important but IMO don't carry nearly as much weight as inbound links (and their associated text) and your title tag.
[edited by: ZydoSEO at 6:10 am (utc) on Feb. 18, 2008]