I recommend you start by doing a Google/Bing search for the term "crowdsourcing" and see what shows up at the top of the organic search results - not the ads. Then perform another search of the site you're interested in joining, including certain keywords like "scam, ripoff, spam, lawsuit, etc...". If a site is shady, it usually shows up in search. Keep in mind that the topic of crowdsourcing tends to evoke some passionate opinions on both sides, so you'll have to filter your results to determine if a 'hit' is an opinion or fact about the site you are researching. If all checks out, then move forward.
Just about any subscription site will make you check the little box that asks if you "agree to the Terms Of Service (TOS) or Terms Of Agreement(TOA)" just before hitting that 'register' button. But how many of us (yes, myself included at times) actually read it? That's a great place to start. Since you are essentially signing a digital contract, it would be good to know what you are agreeing to - before you agree. Pay close attention to the sections that deal with how you will get paid, dispute resolution, and rights to the work you perform/submit. Most legitimate crowdsourcing sites often act as brokers or mediators and will hold any funds until the agreement between parties are satisfied.
If the TOS appear to be acceptable and legitimate to you, then go ahead and register. If you have to pay to register, that's a good enough reason to stop. During the set up process, a good indication of a legitimate site is their request for your deposit preference and account verification. In many cases you won't need to provide that information until funds are ready for deposit, but having a verified account may help you when competing for projects. Having a PayPal account is very useful here.
In the end, your own experience will turn out to be the best gauge of legitimacy. Just be sure to calculate the amount of risk you are willing to expose yourself to, prior to getting into the pool.
Go get 'em.