A question with BA (吧) indicates that the speaker is almost certain of the situation, but not 100% sure. Frequently, adverbs such as 一定 and 大概 can be used in such a question, but 吧 alone can serve the function of indicating near certainty.
nǐ yě shì zhōng guó rén ba?
You must also be Chinese, aren't you?
Also, it is acceptable to use an exclamation mark instead of a question mark after 吧.
nǐ gōng zuò le yī zhěng tiān ， yī dìng léi liǎo ba!
You worked all day; you must be tired, aren't you?
Reference: YUFA! A Practical Guide to Mandarin Chinese Grammar