I'll give you the non-biologist answer....

If you are fishing where there are anadromous fish present, its a Bull Trout.

Without a sample of genetic material it would be hard to tell, except that we are learning about the separateness of the two populations, with the Dollies being present usually above anadromous fish barriers.

One exception to this rule is the Elwha, where artificial anadromous barriers have been erected. Here is the only spot in the park Bull Trout are found above such barriers.

I believe there is a concurrent study being done in the Skagit using very much the same methodologies. I believe the preliminary results of each study support this idea.