If you're talking about fruit in general then the watering rare crop buff is what you want but if you're specifically talking about trees then I think it's just random. I've never seen them either. Some people say fertilizing fully grown trees increases the odds but so far all I've seen is that it increases the yield.

having rare fruit is extremely rare you can't water them so its purely down to rng although im saying this despite the fact i saw a heart cherry in the 1st year so its just luck

I'm still on year two, have seen golden apple, golden coffee bean, jewel almond, chicken mango, all 4-5 star trees just sat on my farm no extra fertilizer just RNG