There are dedicated websites that track the box office numbers of each film, like boxofficemojo.com and BoxOfficePro.com
I should add that box office gross is not always a reliable way to see what will remain popular in the long term. Avatar is the second highest grossing film, but it has been declining in relevance since 2009. Wolf Warrior 2 made over $800 million worldwide, but it is not heard of in the US.
On the flip side, 1959's Sleeping Beauty was a flop for Disney, but it is still remembered as an animated classic. Citizen Kane was also a major flop, but it is often recognized as one of the best films ever.