Hi Xue,
Great question.
Yes, you can tell the difference,
Focal nodular fasciosis is a fusiform swelling of the plantar fascia beginning distal to the calcaneal origin and involving the full thickness of the plantar fascia. The full thickness is the key to differentiating the two entities.
Plantar fibromatosis is also in the mid to distal part of the plantar fascia, although classically it is usually in a more distal location than focal nodular fasciosis. the other features of plantar fibrzomatosis is that it may often be multifocal and finally the most important thing is that plantar fibrzomatosis arises from the plantar aspect of the plantar fascia and there is some deep fibres of the plantar fascia on the dorsal aspect that are not involved in the process. This preservation of deep fibres is the key to differentiating.
So plantar fibrzomatosis does not involve the full thickness of the plantar fascia and focal nodular fasciosis does. You need to look carefully for this sign, but with your eyes it will be easy,
Steve