That is the purpose of the Environmental Impact Assessment (EIA), which must be conducted before any mining is approved. This work typically takes 3-5 years and is performed under an exploration license that does not allow commercial mining. The cost is typically around $30–80M USD and requires many baseline studies with hundreds of scientists. All this data is made public. A regulator will decide if deep sea mining can start commercial operations only after the impacts are known.
For more details on the EIA, see this blog post.
