A little obvious but I always google the name of the product/site and the word 'scam'. You need to be a little careful with this however as a lot of scammers post misinformation in the form of fake blogs or fake comments on other blogs.
I've written a number of hubs on scams e.g. dietary/weight loss/supplement/teeth whitening/work from home scams if that is what you are looking at.
Other than that just bear in mind that if it sounds too good to be true it probably is and always do your research before giving anyone your card details, no matter how small the payment might be. Good luck!