Years ago I asked my grandmother what tribe were we from. Without hesitation she said “you are a Levi !” about 6 weeks later I asked my Rabbi Yonah Geller the same question. He took a month to get back to me and said “Levi.” I am 76 and wonder if this is really so?
Answer:
I don’t know how your Rabbi found this out, but if he claims that you are a Levi, we can assume that that he did the necessary research, and that you are, indeed, a Levi.
Your grandmother’s response can also be relied upon, and her answer without hesitation indicates that this is the truth.
If the question would be whether you are a Kohen or not, the answer might be more complex, but as a Levi there are no major halachos that apply today (apart from being called up for Levi), so that you can certainly rely on these two witnesses.
Best wishes.