Yaourt shows a big fat red warning every time you install a package. It also offers to open PKGBUILD and .install files for inspection.
It should just show the PKGBUILD every time. If it's not doing anything sketchy it's often only a dozen lines.
aurman does a good job. It caches the old PKGBUILD and lets you view diffs. Still, reviewing a PKGBUILD is a non-trivial process.
Thanks! I got bored of looking for a yaourt replacement because they seemed all the same, and discussions of AUR helpers often turn into flamewars, but PKGBUILD diffs is a valuable feature. Trying aurman :)
Yaourt is also unmaintained and unsafe. Please switch to something better.