And, um, on Ubuntu, apt packages are signed, right?
When I think something's already wrong, I use Malwarebytes (free version) to scan my computer. If I suspect something I've downloaded, I use VirusTotal to check it out.
On Linux, there’s no good way to scan for malware because that’s not the focus of most work done on Linux security. Use SELinux (or AppArmor, if that’s your thing), a firewall, and only get software from trusted repositories. Verify the checksums of ISOs for live USBs and use the same sort of common-sense that you would on Windows. Furthermore, you might want to consider moving from Xorg to Wayland since any process running as your user on your display in Xorg can act as a keylogger[0].
[0]: https://security.stackexchange.com/questions/170596/is-it-po...
If I know I have something, usually it's because I permit it through Windows Defender and experimenting with problematic file types, then I use Malwarebytes. I also take a good hard look at my programs lists and remove ones I don't need, review my start up programs, and etc.
- Malwarebytes - Superantispyware - Spybot Search & Destroy
I'm more concerned about something like chrome/steam deciding that it's appropriate to start itself on startup and look for updates than malware that labels itself malware and tries to avoid things like ps. If the sort of malware that avoids ps gets on your system you've already lost.