Can I use a full step up (to my mother's date of death) for a mutual fund that I was a joint tenant of since she was the exclusive contributor to the fund?

I have read articles that since my mother was the exclusive contributor to the fund and she has always paid taxes on the capital gains and dividends herself that the cost basis could be fully stepped up to her date of death.  Would I be required to prove this? I have tax statements for several years past to show her paying taxes on the capital gains  I have also read articles that say only 50% can be stepped up because I am a joint tenant.