Requirements for Fiance Visa

I understand there are several requirements from a fiance visa applicant, such as:

Valid passport
Birth certificate
Divorce or death certificate of any previous spouse
Police certificate from all places lived since age 16
Medical examination
Evidence of support
Evidence of a valid relationship with the petitioner
Photographs (according to specifications)

What forms of documents could be used as evidence of a valid relationship with the petitioner?

Contact immigration I would think as they'd have this info. There should be some info right online here to answer that question more accurately. Sometimes the person has to be actually married to them, before they are allowed into the country. Laws are getting much tougher now and I think unfortunately that is necessary. Too many marry someone in the USA just to get to be a citizen or to stay here, and then later divorce them or cause them trouble financially and lawyers end up with a divorce case to deal with.