Given a stock traded on a public exchange, how can one determine how much of the stock is owned by individual (a.k.a. retail) investors (vs. institutional investors)?
These methods could work globally:
By looking at the annual reports of the company in question. There will typically be a section that lists the largest shareholders.
By extracting the data from publicly-available company filings databases (e.g. EDGAR in the US). Such databases only exist in some countries. You may need to process the data using a computer to actually get the information you need.
By looking at the holdings of many institutions (e.g. ETFs, mutual funds, etc.). You could extract information about the number of shares they collectively own. However, this requires a large database of institutional holdings.
Get the information from a data vendor.
I think you should wait for a better answer, because I am not knowledgeable enough to give you more information.
Insiders are a company's officers, directors, relatives, or anyone else with access to key company information before it's made available to the public.
Form DEF 14A the proxy statement which lists directors and officers, and the number of shares they each own.
Companies file Schedules 13D and 13G to disclose outside beneficial ownership information of more than 5% of a company's stock issue.
Stock owners file Forms 3, 4, and 5 to disclose insider beneficial ownership when they have more than 10% of voting power.
Note that these are not pinpoint accuracy numbers. For example, anyone acquiring more than 5% of a company's stock has 10 days to file Form 13D with the SEC.
Ownership of less than 5% does not require a SEC filing. Figuring out institutional ownership below 5% is problematic. Some analysts use varying techniques to track the limitations of quarterly institutional disclosures. For example, "Investors Business Daily" provides daily and weekly volume information for all listed stocks and has a rating system for purchases and sales. Heavy accumulation implies institutional accumulation.
The NASDAQ has an Institutional Holdings which provides a summary and more detailed view of the aggregated institutional stock holdings, including owner names and ownership analysis.