How does George keep the men from finding Lennie right away?

When all the men at the ranch are preparing to go after Lennie, Slim asks George:

"Where you think he might of went?"

George knows exactly where Lennie would have gone. Lennie would be hiding out at the riverside campsite where they had spent the night before reporting for work at the ranch. George had told him repeatedly to come there and wait for him if he got into any kind of trouble. Although George has a certain amount of trust for Slim, he lies to him here.

"He--would have went south," he said. "We come from north so he would of went south."

George can't trust Slim because Slim is taking part in the search party which is likely to turn into a lynch mob. Slim is the smartest man in the bunch. He may take George's suggestion that Lennie went south, but after the men have gone a short distance without seeing a sign of Lennie, Slim probably would have guessed that George had deliberately misdirected them. This would explain why the mob shows up at the campsite fairly quickly, but not after giving George enough time to get there ahead of them and kill his friend painlessly. George can hear the men from the ranch.

The little evening breeze blew over the clearing and the leaves rustled and the wind waves flowed up the green pool. And the shouts of men sounded again, this time much closer than before.

George and Lennie were traveling south from San Francisco, where they got their job assignments at one of the employment agencies that specialized in recruiting unskilled laborers. George knew full well that the campsite was to the north. His explanation that Lennie would have gone south because they came from the north is very weak and ambiguous. Slim would have quickly figured out that George was lying, but he would not have blamed George for trying to protect his friend.