Letters, proof of cohabitation (bills, rent payments), use of joint financial accounts, photographs, chat logs, wills, written testimonies from friends/in-laws,

Letters, proof of cohabitation (bills, rent payments), use of joint financial accounts, photographs, chat logs, wills, written testimonies from friends/in-laws, a child or three, a five-hundred word life-story essay. Not everyone has all of these things as evidence to a sufficient extent, but most people will be able to provide enough of them to convince immigration. If they can't, if they are lucky, they may get a request for more evidence, instead of an outright rejection. IANAL.