I think he's happy. It's hard to tell.

[kadioguy]

tell
v.
[...]
6 (not used in the progressive tenses)
to know, see or judge something correctly

I think he's happy. It's hard to tell.

https://www.oxfordlearnersdictionari...sh/tell?q=tell
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What do the example sentences mean? (By the way, they are two sentences, right? though I was almost to use "what does it mean?")

Do they mean that "I'm not sure if he's happy, but he does seem happy"? (A friend told me it.)

[Piscean]

it means that it's hard to to know, see or judge correctly whether he is happy, as the dictionary suggested.

[Tarheel]

Kadioguy, say:

A friend told it to me.