K-1 / I-129F evidence of relationship

[Caladan 19]

A rule of thumb:

The purpose of the I-129F is to prove you've met within the past two years and intend to marry. So you need primary evidence (things verified by external sources like banks or governments, like passport stamps and receipts) and some secondary evidence (things that help sketch a picture of the relationship, like pictures and e-mails), but you're not trying to do more than jump the first hurdle.

At the interview, you're proving not just that you've met, but that you're a serious couple. That's the time for more secondary evidence.

The RFE thread has lots of information about how and why people received RFEs. It seems that nothing definitive can be said, since a lot depends on who gets your case, but rough guidelines aren't that hard to guess at.