Translation English Expression "You can tell ... by ...."
The corresponding Chinese expression for 

You can tell ...by ...

is

一看...就知道...

so

You can tell he means it by the look on his face

一看他的脸色就知道他是认真的

Although for a lyric you probably want something like

他的神情告诉我他是认真的

"You can tell the spaghetti is cooked when it sticks to a wall."

能粘在墙上的spaghetti才是煮过的spaghetti

where "You can tell" or "you know" is basically ignored because it won't add anything to the sentence.

"You can tell he's rich by the clothes he's wearing"

"一看他穿的衣服就知道他很富"

"How do you know? (How can you tell)" "I can just tell!"

"你怎么知道的？" "我就是知道"