How can I validate an outsourcing request to tell if it's genuine and worthy of my company's attention?

We have been receiving a number of requests in the form of process management, like back-end data, billing, form filling, etc. offering with high payouts. However, they're asking heavy initial upfront security per seat, bank guarantee. I know most are are fake and one should only trust reputable MNCs, but how do they do business? Are online reviews good enough to judge the quality? Do genuine processes demand security? How to find a genuine process?