Answers referring to ITA and Skyscanner are good, KVS works well too and there are other useful tools. But ultimately, any tool only gives you part of the story. Fares change all the time, conditions relating to them change a lot too, and there are occasional mistakes (e.g. ITA sometimes wrongly think of a fare as first class when it is really discounted economy or premium economy for instance, other deals are not publically bookable or only from a specific point of sale, etc). So if you do want the cheapest deal, one way or another, you need to accept to check regularly, many sites, and do it over and again. There is also no genuine way of knowing if prices will end up going up or down, and that too can of course make for frustrating results.