Translation English Expression "You can tell ... by ...." I'm translating an English song in an exercise to learn Chinese

One of the lyrics is 


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


This is English expression. A less expressive lyricist might say:


  You know he means it because (of) the look on his face


So I tried these translations:


你能确定。。。 因为。。。

You can determine... because...

。。。 就你能确定。。。

... so you know that...



For "can tell" I tried 你能确定  。。。 or 你确定

For "by" I tried either 就。。。  or 因为。But 因为 seems like it's causal. Like "the look on his face is why he is serious". It's the opposite. The look on his face is effect, not cause of his seriousness. The look on his face may be the cause of your knowing he is serious.

For the full sentence I tried:


  你看他脸色, 就确定 他是认真的


How would you translate that line? Emphasis on the Chinese translation for "You can tell ... by ... "

Other English ways it's used : 


"You can tell the spaghetti is cooked when it sticks to a wall."
"You can tell he's rich by the clothes he's wearing"
"How do you know?" "I can just tell!"
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"