How do you know you're dealing with a legit company before you've paid?
Well, I would suggest checking the site before you invest any money in its services. From my own experience, if a website's got:
contact information, different payment options, refund options, information about protection of your personal data, secure payment gateway (https), 24/7 support or online chat, possibility to contact a writer, and... possibility of payment AFTER you receive your paper, it's one of the top writing services. If it lacks any of the above, it's suspicious, so be careful!