What evidence does the old swamper and the description of the text give about the character of the boss?
The old swamper tells the guys that the boss gave the crew a gallon of whiskey last year for Christmas. This must have been quite a bit for the times because this impressed the old swamper and gave him a good opinion of the boss. This occurs in the beginning of the 2nd chapter, 2-3 pages in depending on your book.
There is another interesting thing about this boss. He demonstrates that he is above the rest of the guys by wearing high-heeled boots with spurs, these are boots that you can't work in.
In spite of these two details, the boss is also considered that he "can get pretty mad" too. This is important for George and Lennie to know early on.