In an ideal world all would be as it seems, but since it's not, you will want to try to determine the legitimacy of free offers before signing up. I do my best to do this for you for offers on my site, but the final decision is always yours.

Look at who is making the offer, does it seem like a legitimate company?
Look at the Web site. Is it professionally done or does it look like someone threw it together just to get something online?
Check that the site have a privacy policy that tells your personal information is handled.
Make sure the offer doesn't look good on the surface but appear much less attractive once you read the details.
Determine whether the offer is free of excessive shipping and handling charges relative to its value.
Judge whether more information is requested than seems appropriate for he value of the offer.
Check dates to find out if the offer seems current and does not appear to be an old, abandoned offer that the company never bothered to remove.
If you are still unsure about an offer but are really interested in it you can post a message on the forum. Check the link to the forum under related features below.
Tips:
If you don't feel comfortable with an offer just ignore it.
Don't provide more information than you wish to. For example, I never give my phone number. (555 is a handy exchange)
Use a disposable free email address if you are unsure about the company's privacy policy.