She'll need to contact the firm she receives the checks from directly. If you don't have a phone number, try googling the company to get a contact number
Assuming they are physical checks, start by looking at the envelope and additional information that come with the checks. If there is contact information on any of it, utilize it to send a letter, email or call. Also, she should receive plan information on an annual basis. See if she has saved any of these pieces of information. They will have contact information on them also. If not, ask her to save all mail from the company and you should get something in the next twelve months that has the information.