My niece may have hand, foot, & mouth. how do we know for sure?

Hfm disease : Since hfm disease caused by coxsackie virus, specific blood test can be performed to prove it

Hand/foot/mouth: This is a viral illness with a rash that appears on the buttocks, palms and soles. These may appear as red bumps or bumps with clear fluid. There may be similar blister appearing lesions in the throat. This illness may be associated with fever and a history of contact with the same illness. Either confirm it with a doctor's visit or sometimes sending a picture with a discussion will work.