Another important thing to consider is how you will get the cash and who is paying. If you have joined a mediator type of site, you are usually paid by that site, not the one you are doing the end survey for. If you have joined a database of companies seeking survey participants you are likely to be paid by the surveying company. Regardless of the details you should be aware of the payment method - are you going to be paid by check or by bank transfer etc.
Double Check Your Details!
Finally make sure that your personal details will be used as stated in the contact details you originally submitted. Unfortunately, there are many fraud survey web sites that would offer you to participate in a cash paid survey, for the sole purpose of getting hold of your personal details. From there, your details can later be sold to many other companies and you may end up receiving hundreds of unsolicited mail and SPAM emails. Make sure you investigate the company in which you are going to participate with. Do a browser search on your computer and see if any reviews appear in your search.