You can use google adwords tool or even semrush to look up keywords that a website is ranking for.
With semrush free version you can only see a few keywords that the website is ranking for but if you get the paid version you can see even more keywords that bring traffic to the website.
You can also use similar web and get a few free keywords that a website is ranking for but if you do want to see more keywords you'll need to pay for the paid version to get more keywords.