It's a factual inquiry. There's no magic thing that's sufficient or necessary, though the things you list tend to strongly favor a finding of marriage unless you can rebut them. The core elements tend to be whether there was a mutual agreement (usually implicit) to marry and whether you held yourself out that way. In a dispute both sides present their evidence and a judge decides. Of course in a dispute one side will be claiming there was no agreement. It's not contract law; the inquiry can be far less mechanical.