When you’re logging in to a Starbucks account, booking an Airbnb or making a reservation on OpenTable, loads of information about you is crunched instantly into a single score, then evaluated along with other personal data to determine if you’re a malicious bot or potentially risky human.

Often, that’s done by a service called Sift, which is used by startups and established companies alike, including Instacart and LinkedIn, to help guard against credit-card and other forms of fraud. More than 16,000 signals inform the “Sift...