following the eyeline of players (and sometimes background observers) is the most basic clue, but it's also important to remember that in soccer you're not allowed to touch the ball with your arms or hands if you can possibly avoid it, so when you see a player's stiffened arm you can often guess that they're fighting to avoid an accidental contact that would result in play being interrupted by the referee.
It's not a very useful skill but I hope that soccer gets much more popular in the US as I might be able to win some easy prize money if spot-the-ball competitions become popular. Or for a small fee, I will be happy to provide individual tuition in this potentially lucrative derivative sport :)
Following the direction players are looking is just one parameter and can be wildly inaccurate without any information on the angle and velocity of the ball at the time of the shot. One of the trickiest situations is when 2 players jump for the ball, usually coming from a goalkeeper's clearance, so high and fairly fast, and collide in the air (often closing their eyes): the ball could literally be anywhere.
That's kind of the genius of puzzles like these (we can't take credit for inventing it - Spot the Ball has existed for a long time in the analog world) is that it's a mix of skill and luck, so you can fool yourself into thinking you're amazing at it, when in reality the next photo might trip you up completely.