There is no widely established sign for "prognosis."
A time when you are likely to need to know how to express this concept occurs when an interpreter is interpreting for an individual who is Deaf in a medical, clinical, or hospital setting.
To express this concept it will be helpful to know signs such as:
SEEMMAYBE
LOOK-LIKE
NOT
SURE
PREDICT
WILL-(future)
MEAN-(meaning)
THINK
BECOME
TEND-(tend-to)
Notes: