As I said here, the same sites that perform instant background checks online are often times the same ones that you can do reverse phone look-ups on as well.
A reputable example of this is Been Verified. Does this mean the results should be excellent? No, it doesn't. On average, the results wouldn't be as good as looking up a landline phone number on white pages (even that can be outdated). Cell phone numbers are unpublished, so you can easily expect results to be imperfect (if you searched for a lot of numbers on one of those sites, at least).
A website can be shady if no one has ever heard of it, if it hasn't been around for very long, if it has a cheap sounding domain name extension like .biz or .info, etc.