ahrefs is best at this, short of getting access to the competitor's private analytics or search console account. yes it is an estimate, sometimes out of date and sometimes over or under estimating by multiples of 2 or 3, but it's a good ballpark, it's usually accurate within 50%, very often accurate with 20%, and is very rarely off by a factor of 10. obviously it's focused on traffic from google - direct traffic and referred traffic from other sources, especially non search engines, is a very different story.