Instructions
Wander around fighting wild Pokemon. Most wild Pokemon live along the routes between towns, though you can find others in abandoned buildings and caves.
Go to a town and speak with the nurse at the local Pokemon Center. If one of your Pokemon contracted the Pokerus, she'll inform you. If she doesn't tell you about the Pokerus, then none of your Pokemon are infected. Return to wild and fight more Pokemon.
Fight as much as you can with the infected Pokemon at the head of your party. Every battle that you complete with the infected Pokemon nets double the normal amount of evolution points. Unfortunately, the Pokerus eventually goes away, typically within a day or two; take advantage of it while you have it.