To find clear examples of Conrad's racism, it may be helpful to remember that Conrad's view of the Africans was typical of the European and American attitudes and prejudices at the time.
Think about the various beliefs which were parts of that prejudicial view: the belief that Africans were uncivilized savages, that they were incapable of abstract and highly intellectual thinking, that they were predominantly ruled by their emotions and "animal" instincts, etc.
When you have those beliefs clearly in mind, go back through the book, looking specifically for instances when Marlow seems shocked or disturbed by the presence and behaviors of the natives. Other characters also refer to the African natives as "niggers" and as "devils," but Marlow seems to have more compassion for them than those other characters. Even with that additional compassion, Conrad's prejudice comes through.
In chapter one, you might try looking for the reference to a line of men in loin cloths and baskets on their heads, which is just after Marlow has first seen the mouth of the big river. Conrad's build up to the encounter with the cannibals and then the description of those men and Marlow's reactions to them in chapter two provide examples of these prejudiced views; though Marlow states that he "looked at them as you would on any human being." Near the beginning of chapter three, Conrad's descriptions of the men carrying the man on a stretcher show some of his racist attitude. He uses many words with very dark and sinister connotations throught that passage.
Racism is evident in his treatment of the more "civilized" African men, too, such as the "savage who was the fireman" on the steamboat and the "fool helmsman."
I'm sure that if you pay attention to the language used to describe these minor characters, you'll have plenty of examples of Conrad's racism. A literature guide like enotes or Sparknotes can often help guide you to the right sections, as well.