Well, I think the lesson that Earth teaches is this:

You could send a small number of drones to Earth, and perhaps none of them would detect life.

For example, the probes would hit ocean water, and sink rapidly, unable to send out a signal, since Earth is like 70% plus an oceanic world.

And then maybe some of your probes would land in a desert, or Antarctica and have difficulty detecting life.

But if you instead sent a whole swarm of cheap probes, like tens of thousands of probes, or even millions of probes, you'd detect life on Earth instantly, with even the most basic chemistry-testing experiments, or basic microscopes.

So... ya, at this point, I think we've got a handle on "We'll know life when we see it!" in terms of basic chemistry.

Thus probably just best to send a huge swarm of simple probes to worlds, rather than overthinking overlycomplex probes.