Life outside of Earth: Wouldn't it be safe to say that our landers and probes transfer microbes?

It's certainly possible. The satellites are built in clean rooms that go to every extent to prevent that possibility, but nothing is perfect. However, if we DID find microbes on the other planets we've visited, it should be fairly easy to determine if they came from Earth - they'd be structured similarly to Earth microbes, and share the same DNA. Of course, that could just mean life was seed on both planets, but that's not as likely as it being our fault.