Rayleigh already explained it, everyone has Haki.
Being able to use it is something that only talented people will really exhibit.
You can have Haki, but you may never be able to use it even if you train your entire life.
It's simply impossible to know what Haki someone can use until they use it.
Mihawk believing Zoro can surpass him simply means he sees him as someone with potential. Seeing someone as having potential doesn't mean they'll live up to that expectation.
Many footballers have talent, but how many of those talented footballers end up becoming the best in the world? Only a few.
All Zoro did was prove his character as someone with strong ambitions and pride, whether he'd really pull of becoming a challenge to Mihawk? It's pretty much the same thing Shanks did.... he took a gamble and he's seeing if it will pay off in the end.
You just can't know what Haki someone has until it's been showcased.
- There was never a reason for:
- Garp to know what Haki Luffy had.
- Mihawk knowing Zoro would be a CoC user.
Knowing what Haki someone would have is pretty much asking for a character to be psychic, only character that remotely even pull this off isn't even a Haki user, it's Madam Shirley.