how can scientists ensure that advances in medical science and technology will not be harmful to environment?

Answer 1

they can only watch for effects but they can predict these effects with a great deal of accuracy based on previous data

and you can't prove a negative but you can assign a probability based on data and that is what science is all about

you can't prove that gravity will exist tomorrow but you can lay something down with the almost infinite probability that it will not float away because you have data that says so

Answer 2

Scientists cannot ensure that, that is where the Government steps in to determine the balance between technological advancement and environmental balance.......tragically the balance is not always right.

Answer 3

This would require them to 'prove a negative'. Unfortunately, you cannot prove a negative. So, the only possible answer is 'they can't'.