I honestly wasn't suspicious until after I met the mother. She creeped me out in all number of ways. I scrutinized the whole farm, looking for any number of reasons to not trust them. The only thing I found suspicious was when I knocked on the door, and talked to Brenda again, and asked her how many rooms the house had - she immediately got defensive (I honestly didn't think it was that weird to ask someone who had offered us to stay with her how many rooms she had).
Andy's comment about hearing us "yell" rang SOME alarm bells, but then I thought it could have been possible that he heard the bandits yelling. At Jolene's camp, Danny seemed honestly relieved that the boxes from the dairy were empty (why?), and also that the camcorder didn't work. Then talking to Jolene, I paid careful attention to her words, and Danny shot her before she could say what she knew the farm family had been doing.
I was definitely not trusting them now. The first thing I did back at the farm was ask to see Mark (was told no). I talked to everyone else that I could, and then snooped around the barn. I found the pile of dirty clothes first. Then I opened the barn door and the first thing I asked after being caught by Andy was "Why is there so much blood in there!". From there it was only natural to continue snooping inside the house, and to try to find Mark.
Now, one of my first thoughts was that they were cutting up people to feed to the bandits (a lot of emphasis was put on the "deal" they made to keep the farm safe). It wasn't until I thought about the "special dinner" comments as I was searching for Mark that made me think that they were cannibals.