Is there anyway to check what impact (removing) a Frontier Station has on my borders?
Solution 1:
While there's not a good way to check what will be removed, Frontier Station borders both do not grow, and, allowing for +border expansion techs, will always be the same size.
So while you won't be able to get stellar information about what you will lose, the more you play you should have a better chance of "eyeballing" it.