The referendum in Crimea has been repeated by numerous Western organizations, including Gallup. [1] The results are nearly identical to what Russia said they were. Russia claimed a 97% "yes" with an 83% turnout rate. Gallup found that 82.8% of Crimeans agreed that the poll reflected the will of the people. It's two ways of saying, more or less, the
exact same thing. The minority that opposed joining Russia chose to boycott the referendum, which is why it was 97% yes instead of ~83%. It's going to breakdown largely on ethnicity. Crimea is about 68% ethnic Russian, 13% ethnic Tatar (which is an autonomous republic within Russia), and then about 16% ethnic Ukrainian.
If you're looking to history - this event mirrors Kosovo, just with roles reversed. An ethnic minority, geographically isolated, within Serbia declare their independence. This is rejected by Russia and backed by the US who ensures forces are on the ready to "enforce" Kosovo's independence. The big difference is that the one remained relatively peaceful with no further largescale escalation. Ukraine, by contrast, was determined to retake Crimea and clearly had the full backing of the West. So, in many ways, this war started 9 years ago.
[1] - https://www.forbes.com/sites/kenrapoza/2015/03/20/one-year-a...