I really would not have thought of the polar bear so much as an indicator species. I suppose it is how you use the term. In the field, plants often a much better indication of the type of communities. Here in FL for example, bald cypress is a good indicator of a swamp forest, pines of a sandy dry upland --- only to a point mind you and so on. You have nice easily seen nonmotile species that indicate the conditions for a certain kind of community.
Using an apex predator as an "indicator" is rather tough. Apex predators are awfully rare. They are also highly motile and have large ranges. Apex predators generally range over a number of communities. They need to eat so much that they can not really be tied to a single type of community. However, human intervention usually messes up apex predators early on so their presence is a decent indication. Think of it as diving and if you see large grouper... the reef is probably well protected because otherwise they would be fished out.