I look at the latest release date. At leisure time, I would also go and check repository and issue tracker to see whether something serious is being ignored. If it's crucial for business, I would spare time investigating the source code itself.

I would not necessarily say that many apps uploaded to F-Droid and other repositories are unsafe, because I don't have all that energy to audit anything I use. What helps me to stay on the safe side is reading into things - enclosed descriptions and names may look like a small factor to some, once they tread the sources, but it saves me both the time and trouble. Sloppily written stuff usually implies a sloppy code, a lax attention to details on the developer's side.