My father has recently passed away, my mother and father have been together since 1989 but have never "officially" married but lived as husband and wife my dad collected social security benefits and my mom collected SSI. WE would like to apply for "spousal benefits" for her to help her continue to pay her bills what would I need to have to prove that they have been together all this time?
I'm sorry for your loss.
The fact that your parents lived together for a long time wouldn't necessarily entitle your mother to survivor benefits on your father's account. In order to potentially qualify for widow(er)'s benefits, a surviving spouse must have been legally married to the deceased worker for the requisite period of time mandated by Social Security law, which is normally a minimum of 9 months.
There are states that recognize common-law marriages, but in order to meet those requirements both members of the couple must have had an intent to marry in addition to meeting all of the other legal requirements. You may want to check out the following reference from Social Security's operations manual to see if your parents may have met the common-law marriage requirements in any state(s) in which they resided: https://secure.ssa.gov/apps10/poms.nsf/lnx/0200305075.
In order to prove that a common-law marriage existed in states where they are recognized, Social Security accepts various types of documentation such as mortgage or rent receipts, insurance policies, and medical and bank records. And in order to prove that there was an intention to marry, they require completion of form SSA-753 (https://www.ssa.gov/forms/ssa-753.pdf) by the surviving member of the couple and by a blood relative of the deceased worker.