Most AUR "helpers" show you the PKGBUILD and/or a diff thereof and ask you to confirm that it looks ok before continuing the install. At least by default.
Maybe most users just ignore that and always answer yes without inspecting it. I don't know. But the wiki for the AUR and Readmes for many of these tools have warning banners telling you not to blindly trust AUR packages.
There are subtle things like abusing how github handles forks which can make malicious PKGBUILD a matter of just changing the rev with no hint in the file itself.