Check the Domain: The very first clue you're going to want to follow is the domain. A domain is the part of the URL that specifies what exactly that site is part of (institution, government, business, etc.). For example, if the email address you're looking at looks like this:
You can see from the domain in this email address that Bill is affiliated with something called "fireplace.com". Using this clue, you can navigate to the "fireplace.com" Web site (or whatever website your domain is affiliated with),and do a site search for someone named Bill.
Google it: You'd be surprised at how many times I've simply used Google to find out who an email address belongs to. Copy and paste the email address into the Google search field, and if this email address is printed somewhere on the Web (on a Web page, a blog, a social networking site, etc.) then you'll hit paydirt. While you're at it, I highly suggest using more than one search engine in your search; you'll turn up little bits and pieces with each different search tool.
Use the email: Sometimes the easiest solution can be the best one. If you're not sure who that email address belongs to, simply email them a polite message asking for their information - it couldn't hurt to try, anyway.
IP address: This one is a bit more complicated than the three I've already mentioned. An IP address is a series of unique numbers that identifies a computer connected to the Internet. Every computer that gets online has an Internet address, and most of the time (not always), you can search the header of the email you've received to get it. Once you have that IP address, plug it into a simple IP address lookup tool, and you'll be able to determine the general geographic area of where that email originated.
Email addresses can't always be found, so be patient. Not everyone publishes their email and other personal information on the Web, so keep that in mind as you pursue who an email address might belong to.