Probably, must be: chắc (là)

Chắc is the everyday way to make a guess you're fairly sure of, like "probably", "I guess" or "must be": Chắc ảnh quên rồi. He's probably forgotten (in Hà Nội, anh ấy). You'll hear it all the time in Saigon.

It usually opens the sentence, often as chắc là, which means the same: Chắc là em tới trễ. I'll probably be late. It can also go after the person, right before the verb or describing word: Chị chắc mệt lắm. You must be exhausted.

As a reply, Chắc vậy means "I guess so" (in Hà Nội, Chắc thế), and Chắc là không is "probably not".

The same word also means "sure". Em có chắc không? asks "Are you sure?", and chắc chắn is "definitely". The place tells you which: before a sentence or a verb, it's a guess.

Có lẽ (perhaps) is a softer guess and is common in writing too. Hình như (it seems) is for something you've heard or half remember. Chắc is the most confident of the three, and the most spoken. Hẳn là and chắc hẳn are surer still and more careful, common in writing: Hẳn là anh ấy đã biết. He must have known.