I want to use a recent package from a PPA. How do I make sure that the PPA maintainer hasn't added any malicious code in the version provided there?
Since PPAs are built from source on Canonical's infrastructure, you can trust that the binary you have installed from the PPA matches the source that you review as above, provided that you trust Canonical. This should be acceptable given that you trust Canonical to build Ubuntu in the first place.
You really can't be sure there is no malware in the package provided by the PPA, but you can at least check that the source code hasn't been tampered with:
The two source trees should be identical. If they aren't, you can go through the differences in detail and make sure they're not malicious.
You also need to check the packaging itself. Debian packages can contain patches to the source tree, and they may add additional code.
Debian packages (.deb) can be unpacked like so: