tell from是区别,辨别,认出的意思；
tell by是通过什么认知到什么的意思.
can you tell Tom from his twin brother?I can tell by their smiles.这是很典型的例句,意思是你能分辨出Tom跟他的双胞胎兄弟吗?我能通过他们的笑容分辨出来.