There are several editions of Of Mice and Men. One of your best bets is going to be to look through chapter 2. This is when they arrive on the ranch and Candy, the swamper, is the one who sort of shows them the ropes...
I would imagine that if you skim the first 4-5 pages of your copy, you will find the exact quote or instance you might be looking for. The text describes Candy as a bit older, and a handicapped man who cleans the bunkhouse among other places.
There are two mentions of Candy's being one-handed. The first one is described in Chapter 2 in the fourth paragraph (page 18 of the Penguin edition)--
He pointed with his right arm, and out of the sleeve came a round stick-like wrist, but no hand.
Then, in Chapter 3 in the third paragraph on page 60, Candy's loss of a hand is also mentioned--
He scratched the stump of his wrist nervously.
Because Candy lost his hand on the ranch, the boss compensated him with two hundred and fifty dollars and gave him the job of swamper. Nevertheless, Candy is always anxious about his position; for example, when he first enters the bunkhouse in which George and Lennie are talking, Candy is quick to assure them,
"I wasn't listenin' I was jus' standin' in the shade a minute scratchin' my dog. I jus' now finished swampin' out the wash house."
In another instance, after Candy tells George and Lennie about Curley, he asks them, "Don't tell Curley I said none of this. He'd slough [fire] me...."
Further, as he listens to George and Lennie discuss their dream of owning a place of their own one day, Candy becomes eager to be a part of this plan. He offers to contribute his savings and even promises to make a will that leaves his money to them. Scratching the "stump of his wrist nervously," Candy adds that he thinks he will be fired soon because of his age:
"Jus' as soon as I can't swamp out no bunk houses they'll put me on the county."
Candy's physical handicap becomes symbolic of the crippling state of all the dispossessed men who migrate from place to place in search of seasonal work with little hope of anything permanent in their lives.
On page 18 in my copy of the short book "Of Mice and Men" George and Lenny arrive at the bunkhouse. They are greeted by Candy who is cleaning the bunk. Candy is described in the book as being an old man. He points with his right arm. It states that out of his other arm came "a stick like wrist, but no hand."
Candy then tells the guys where hey will sleep. George finds some strange yellow can and asks Candy what it is. Candy tells him that the last person had it and it kills lice.