Non-Objecting Beneficial Ownership (NOBO) lists.
If a NOBO list turns up shareholder claims to something like 15b shares, while the total O/S is something like 5.63b, then the CEO can be pretty sure that they have a significant short position.
The individual brokers with short positions can be determined by comparing their NOBO listings to their DTC listings. The broker's NOBO listing would provide information about the number of shares that the broker has sold to its shareholders. And, the broker's DTC listing would provide information on how many shares the broker actually owns and has certificates for. Therefore, the comparison would be shares sold versus shares owned; which should reveal the number of shares that the broker sold short, if any (apparently, unless the broker simply refuses to comply with the NOBO listing).
Other indicators can be things such as a PPS that continually plummets over time, regardless of whether or not the company has good news or positive developments. However, in most cases, this actually occurs because the associated company is massively diluting.
