This JQL function can be used for searching for users by checking against user properties that contain numeric values.
usersWithNumericProperty(propertyName, operator, number) #Output: User list
If you want to search for issues where a user as a specific text or date property value, have a look the the usersWithTextProperty() or usersWithDateTimeProperty() function.
If you want to search for issues, checking only if a property is being set ignoring it's value, have a look the the usersWithProperty() function.