Basically a fused bloom is two buds that have fused together and the blooms will have more petals and sepals than normal. If there are two pistils and 12 stamens then it is a fused bloom, there may be less than 12 stamens sometimes though. The base of the bloom will look like different as well.
A poly has more petals and sepals on one bloom, will have one pistil, and it will have the same amount of stamens as the petals and sepals. Sometimes there are 8, 10, or even 12 petals and sepals. I have had some that just have 7 so it didn't fully poly. You can tell if a bloom will be a poly before it opens because the bud will have more segments.
