Here are two general "rules of thumb" that should keep you out of trouble when searching for a legitimate work at home opportunity among the expanse of scams:
You should never have to pay them anything, period.
Any mention of you sending them money for any reason indicates a scam. This is doubly true if they offer to send you money up-front and want a part of it sent back or to someone else.
Don't re-mail things for them
If they want you to accept shipments and send them back out, or re-sell them on e-bay, etc. don't do it. This is usually a front for fencing (laundering) stolen property.
If they are too eager to get your SSN, run
Many frauds act like they have a job and just need your information to fill out tax forms. This generally is NOT needed for contractors, but can be legitimate if they are hiring you as a full time W2 (US) employee. However, you really really need to check a company out and make sure it is legit before you give up this information. A red-flag is a company that asks for personal information too quickly, or doesn't seem interested in discussing other details of the work. These job scams are a major source of identity theft.
Some legitimate places to find work at home jobs:
There are a number of online sources that connect people who want to do freelance work to companies that need the help. A major benefit is that many of these offer some kind of vetting process for both ends of the transaction and an escrow system to arrange the payment without giving up too much sensitive information. The downside is that the sites often take a cut of the deal (5-10% or so). Here are a few I recomend
Pros: Larger jobs/more money potential
Cons: You will probably need to spend more time setting up a portfolio and may need to charge less until you build a reputation on the site to get the really good jobs.
Typical Jobs: More skilled jobs like writing articles, building a web site, graphic design, etc.
Amazon Mechanical Turks Program
Pros: Easier to get jobs without much prep or sales work. Also, a lot more less-skilled jobs are available here.
Cons: Jobs are usually really small, and pay a usually small piece rate.
Typical Jobs: Proof a web-site or document for errors. Visit a web-site and post reviews on other web-sites.