This is my perception, and it's not that different from what Papa Chicken is suggesting...
There isn't a discipline known as environmental science. There are scientists using disciplinary tools from multiple disciplines and applying them to specific problems.
As a result of this, rigorous training in any of the major science disciplines will not only make you a more versatile scientist/candidate, it will not only make it more likely that you learned proper depth and skills in an area to be an effective scientist, but it will also make you just as prepared as someone who specifically studied environmental science because you'll just be applying a translatable skill to a specific set of problems.
At Brown, we offer both Environmental Science (which is a physical science concentration, an Sc.B. is awarded) and Environmental Studies (which has a science and policy component).
I guess one very basic check is whether the school is offering environmental science as an Sc.B. and whether they're counting it amongst their physical sciences or not.