How are blanks indicated for placeholders in Chinese (like ???) In English, in the middle of a sentence if data is being left blank (such as an unknown date, or a name to be filled in at a later time), commonly you see ???, ___ (underscore(s)), or -- to mark this. You can see examples in books such as Mary Shelly's Frankenstein, where the year is always indicated as 18--, hiding the last two digits. In Pokemon Gold, Silver, and Crystal (although not the most formal example of language), your rival was called ??? until you named him.

In Japanese and Korean, 〇〇 (or simply 〇 at times, though this is easy to confuse with 0 in Japanese) is used to mark "something" in that place.

In Chinese, what is/are the most common ways to suggest that data is missing or incomplete?
Generally putting XX is fine unless formal. People use that a lot orally. X is usually pronounced as 叉, but can vary based on region.

Formally and also very commonly for missing name is using 某. 

王某 (someone with surname 王 and one-character given name)

王某某 (someone with surname 王 and two-character given name)

某某/某某某 (very general, someone with unknown name, no implication on length of name)

For dates, if exact year is unknown or of no importance, you say 19世纪 (19th century), 19世纪80年代 (1880s)