How can I differentiate between the scams and the legitimate jobs?
Beyond that, do a lot of homework. Do several web searches on the company name, and any variations of the company name you can find on their site. If you find NO results, this is a red flag - they are either too new, or have just changed their name to hide a disreputable history. If you find just a couple of bad results, dig further with the BBB and more searches. It could be a red flag, OR, it could be their competitors trying to drag them down.
Read your search results carefully as well. It's quite easy to get a lot of phony reviews written, so watch for things like duplicate reviews, glowing, scripted reviews that sound just a little too phony and polished, or a pattern in the way the reviews appear.
Read guarantees carefully as well. Some places offer a "money back guarantee" only to find that they will keep a $150 processing fee.
Also read up on common scams such as payment receiving services, in which companies find individuals to take payments for stolen (or nonexistent) merchandise.
In the end, if you are really interested in a company, pick up the phone or send them an email. If you can't find a real person to talk to, move on.