In message .com, yue
writes
Could anyone let me know how to tell the difference between a dill and
fennel plant. I think I have one in the garden with feathery leaves
and is about 6 feet tall. I checked some books in the library for both
plants' ID but I still can't tell the difference. Please advise.
Thanks in advance.

Apart from the odour as mentioned elsethread, Stace describes some
differences in the stems and fruits

Fennel - stems solid at first, becoming +/- hollow; ...; fruits scarcely
compressed, c. 2-4x as long as wide, ..., with prominent thick ribs.

Dill - stems hollow; ...; fruits strongly dorsally compressed, c. 2x as
long as wide, ..., with prominent slender dorsal ridges and
conspicuously winged lateral ones.
--
Stewart Robert Hinsley