In Chapter 2 of Of Mice and Men, how does Candy know that George and Lennie actually came to work?
All the evidence that Candy can obtain (from talking to George and Lennie and observing their actions) suggests that they have arrived to work.
While Candy is uncomfortable that George is doing all the talking (why isn't Lennie involved?), all the information provided suggests that they are here for work. Even when Candy overhears George say he is glad that he is not actually related to Lennie--in conflict with the conversation they had just had with the boss--there's nothing to suggest that they won't work. In fact, the boss is satisfied with their previous work experience and (false) reason for leaving their last employment.
George and Lennie are a day late, but they have re-affirmed their work ethic and there's no evidence to suggest they are at the ranch for any other reason.
There are also more subtle signs, aside from direct conversation. For example, both George and Lennie make their beds. George has already voiced his disgust at the lice-powder and poor-housing, but nevertheless they unpack all of their belongings and George makes his bed 'neatly with blankets.' Lennie follows suit and also makes his bed. The men could just bunk down for the night, with their personal belongings packed ready to go. But George makes a point of unrolling "his bindle and put things on the shelf, his razor and bar of soap, his comb and bottle of pills, his liniment and leather wristband." Making the beds and 'setting up home' convinces Candy that they're staying.