If you have eyes, all you need is the trinket. Here's how you do it.
1: Look at where the enemy is on the minimap.
2: Subtract those present elsewhere.
3: Add those with global movement ults (Kench, Ryze, Pantheon, Fate, Sion, Kled, etc.)
4: If Karthus exists, add him as a possible factor.
5: Secure your exit strategy.
6: Identify where the missing champions might approach you and how.
7: Position and move yourself to make that approach harder for them.
8: Be aware of your lane opponents' movements. If they start being super aggressive, RUN.
9: If you are alone, run away twice as fast.
10: Run through 301,139 simulations per second in your head. If you come up with a failure rate of 60% or higher, then hostile outside forces are approaching and you should fall back IMMEDIATELY.
But... I can tell you right now, the sightstone isn't an instant win/lose condition. If it was, it would have been removed long ago. I've seen more games than you could ever hope to imagine lost due to not paying attention to the wards players keep bitching about, autotilting Vaynes with a Soraka trying to tower dive at level 1, never pushing, defending or going for objectives, trying to endlessly teamfight for the elusive pentakill, not bothering to dodge attacks, not doing anything outside of a full entourage of five, and ragequitters. Very rarely have I ever seen a case where the problem was a lack of vision, and that's usually attributed to an auto-attack assassin trying to chase the enemy team into a bush.
