It is legal for a person to cash someone else's check if the check owner signs the check over to that person, reports About.com. However, it is illegal to cash someone else's check by signing another person's name. This is the crime of forgery, which is potentially punishable by fines and jail time.
Someone legally signs a check over to another person by signing the check and writing "pay to the order of" and the name of the person to whom they are giving the check, explains About.com. Although the person can then legally cash the check, the bank may have policies that prohibit this. Before the original check owner signs it over, the person receiving the check should verify that her bank permits it.
Signing another person's name on a check constitutes the crime of forgery even if the check owner is a child or an incapacitated parent, according to Bankrate. Power of attorney is necessary to handle the finances of someone else. Penalties for forgery vary by state and depend on whether the person committing forgery intends larceny or fraud and whether the state classifies the forgery as a misdemeanor or felony, points out About.com. In some states, the forging of financial documents is a felony subject to heavy fines and a long prison sentence.