The Book of Acts
1) The internal writing style in the Gospel of Luke is different in some respects when compared to Acts. This tends to indicate that someone else wrote Acts. For example, note the use of the Greek word τε (9 times in Luke; 151 times in Acts). Also compare the difference in use of ανηρ (27 times in Luke; 100 times in Acts) to ανθρωπος (95 times in Luke, 46 times in Acts). See the following article for other examples: A.W. Argyle, "The Greek of Luke and Acts," New Testament Studies (Cambridge University Press) 20 (1974): pp. 441 - 445.
2) Contradictions to other events in the New Testament. For example compare Galatians 1:11-2:10 to the corresponding events in Acts.
3) Significant amounts of material which are unsubstantiated. Many of the details found in Acts are not present in Paul's writings even when they would be expected. For example, compare the details of Paul's conversion and persecution of the Church in Galatians 1:13-24 and 1 Corinthians 15:1-9 to those found in Acts.
4) Similarities of some parts of Acts with Greco-Roman literature. This may indicate some of the material in Acts was created based on stories in the literature of the time. For examples of this see: Ruben Rene Dupertuis. "The Summaries in Acts 2, 4, and 5 and Greek Utopian Literary Traditions." PhD diss., Claremont Graduate University, 2005.