With LinkedIn, you can suppress your contacts (which they call “connections”), so other people can’t see them all. You can see what the individual posts and see other folks’ comments on them, so that helps give you a sense of how legit they are. You also can see the number of followers they have. Typically, folks with legit accounts will have at least 500 connections — each of those people have chosen to connect with them. And you can see whom they have in common with you, connection wise. Depending on what they do, you also might see their portfolio or work samples.
I find it dead easy to check someone out on LinkedIn, because even if they suppressed all contacts, you can see their employment history. And you can easily search for people who work there as well or worked there during that time. You could easily contact those people on LI. To lie effectively on LinkedIn is typically hard, because anyone who wants to do due diligence can do it pretty easily.
If someone’s profile is skimpy, then it won’t be much use. And in some industries if that’s the case, that might be a red flag. Like to work in the tech, startup or venture capital world and not have contacts in common and easily checkable is rare for legit people.