If I'm unsure of the source of a pleasant scent, what should I bless?

When uncertain about the source of a pleasant scent, the blessing 'Borei minei besamim' is used as the safe default. This follows the general halachic principle that when in doubt, we choose the more general blessing, which covers all possibilities. This way, the blessing is valid regardless of whether the scent comes from a tree, herb, or synthetic source.