So the question is, how does a person know if he has to make a Borei Atzei Besamim (Who creates fragrant trees), Borei Isvei Besamim (Who creates fragrant herbs), or Borei Minei Besamim (Who creates various kinds of fragrances)? The Ben Ish Chai gives a siman (mnemonic). He says that if the stem is hard, it's Borei Atzei Besamim. Also, it has to last from year to year. These are the simanim we have; those are Borei Atzei Besamim. However, if the stem is soft and it does not last from year to year, that's Borei Isvei Besamim. Some say that if it's soft and it lasts from year to year, then it's Borei Atzei Besamim.
So, what happens if you're a mesupak (in doubt)? If a person doesn't know what to do, the Ben Ish Chai says, Borei Minei Besamim. So again, if the stem is hard, it lasts. Atzei (trees) is Atzei, Borei Atzei Besamim. The tree has to last from year to year. So there again, Atzei Besamim, if it is hard, if the stem is hard. If it's soft, it's Borei Besamim (Who creates fragrant plants). The tree has to last from year to year. However, if it doesn't, then if you're not sure, Borei Minei Besamim. Just like Shehakol (Everything exists by His word), when the Beracha (blessing) is a safek (doubt) on eating.