Spotting a online scam can be tricky, as many of them appeal to certain things in our lives that we just cant ignore. The prospect of free money, or secret information is just too good to miss for some people.
The popular TV show "The Real Hustle" uses a phrase that is so true when it comes to spotting scams both on and offline.
"If it is too good to be true, it probably is"

Adverts that promise free ipods, or free money etc - are usually (if not ALL) a load of rubbish, and the adverts are designed to encourage you to click on them so that the advertiser gets more traffic to their site.
If you have read the page about Relevant text Ads, then you'll understand why people want more traffic to their sites.
Other scams can be cheap items on eBay, that have misleading descriptions. In our time on the internet we have seen phone BOX's being sold, with a description that suggests there is a phone in the box - when in fact their isn't.
Our advise to you is to read things 3 times, and if at any time you are required to hand over money - walk away.