When looking for freelance online jobs, you'll often see no pay or very low pay jobs with the promise that once the company makes it big you will be first in line for the big paying assignments. This is akin to banking on your neighbor or relative remembering you when they make it big time or become famous.
First of all, most likely they'll never make it to the big time money and jobs. It's just a sad fact that not all new companies make it, and that those who do probably won't be generating huge revenue for at least years.
Another red flag of little return on your work is when you are promised exposure for your work. If this promised exposure comes with decent pay, it might be good work. However, if the exposure is the pay you are likely getting ripped off or sold short for your work.
Remember that if you are getting started you may take some gigs now that you wouldn't dream of years from now, but that doesn't mean you have to work for free.
It seems that since all literate people can write to some degree, many people feel that everyone can write. Therefore it isn't a valuable skill. This is far from true. Most people can speak, but can they speak persuasively to a crowd? No. It's the same difference. There are many niche writing skills; not all writers are masters of all styles. An ad copywriter might be a horrible fiction writer; a fiction writer may be an awful technical writer.
Search writing companies, websites, and jobs in Google or another reputable search engine. If they've been scamming people or not paying for work rendered then you know that they aren't worth the bother. If there are few negative comments among the sea of good comments then you know that the company is probably reputable. You can also check them out in the Writer Beware forums and the Better Business Bureau if you are still hesitant about working with them.