Guys, focus, this isn't about how they got there.
You can determine the chemical components of the air/water/soil from samples (Not my field, but I think it would be a mass spectrometer.). I have no idea how one tests for gravity other than dropping something and seeing how long it takes to get to the ground, but I'm sure if you asked physics people there's an equation for how much gravity a thing of particular size has. (I asked Dr. Google, who says it's Gravitational force = (G * mass1 * mass2) ÷ (distance^2), where G is Newton's gravitational constant. For most texts G = 6.673×10-11 N m2 kg-2.)
When you get to the 'But what about Horrible Unknown Alien Disease, personally, I'd go with frogs. Their skins are incredibly porous, which is why frogs are often the first symptom of pollution here on earth. Plus, some species of frog can just be frozen and then thawed out and be fine. (How remains a super cool science mystery, which is probably not relative to what you're doing.) Of course, sometimes what causes cancer in rats only causes cancer in rats, or never causes cancer in rats but always causes cancer in Capuchins. In the end, the only way to be 100% is not one person being potentially sacrificed, but many, many people being potentially sacrificed. Different things affect different people differently based on factors of sex, race, overall health, and in certain memorable cases, a random genetic quirk that doesn't seem to really do anything except mean you won't get AIDS.
If none of this is lighting your candle, though, you could always go the Star Trek route of having the Scientist!character wave a Doohickey around and loudly declare that everything is fine, and everyone can take their helmets off. It's light on sci and heavy on fi, but it's an answer if you don't have a better one.