They probably don't. Using ones that are open source is a good start, and sticking with big names/reputable devs is a good idea. Mozilla also does some checking, but I don't know how extensive the checks are. As with all software, you need to be cautious with what you pick.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man.