I'd start by contacting that employer and asking them. Barring that, you can search this website to see if your former employer ever sponsored a plan. Go to the homepage and in the top left enter your employer's business name. Voila!
One note to Erik's answer, which is bang on. For now, you can't search under the Q&A tab, even though the dropdown shows up. You can't click on it. So, if you're looking at this answer, you can't just go up to the search box and search from there. You'll have to go to http://www.brightscope.com/ratings/ or, as Eric mentions, the home page, and then search from there.
Hopefully you found your answer by now but if you did not, go back and look at your Tax records from the time you worked there. On your Tax forms W2 specifically, look for box 12 and 13 if there is a check in 13 then go back to 12 which will have a code and more than likely an amount that was contributed to your plan for that year.
Thanks for your submitting your question. Brightscope is an aggregator of retirement plan information however does not handle your employer or former employer’s retirement plan specifically. If you still have the contact information of the person who handled anything to do with human resources at the company that would be your first place to start for the information you are looking for. Additionally if you run into further setbacks here is a link to an article I wrote on tracking down a 401(k) http://www.brightscope.com/financial-planning/advice/article/24529/10-Ways-To-Find-Your-Old-401K/ I hope all of this helps. Feel free to reach out directly or contact other advisors who post an answer to your query as I’m sure one of us can help. See tax assistance as well to understand the rules there if you make any transactions on your account.