[BurningFrog]

Pour in some traceable mix of chemicals, and see if it turns up somewhere.

[tuatoru]

There're these fish called the Devil's Hole Pupfish. Can't do that.

[BurningFrog]

There is a near infinite number of non toxic chemicals to choose from.

[tuatoru]

Ah, but can you show that they're not toxic to Devil's Hole Pupfish? And the population is tiny and fragile--you're not allowed any test subjects.

[BurningFrog]

There might well be 5 separate regulations forbidding it. That's a different question.

The safest class of substances I can think of are unusual isotopes.

They're chemically identical to the regular elements, which is what matters for life forms, but can be detected by scientific instruments.