[Note that just because a letter was not written by Paul does not mean that it is necessarily a forgery. The letter might, for example, have been written by one of Paul's scribes or associates and reviewed by Paul himself. For a great summary of this issue, see Ben Witherington's post here.]
The arguments against Pauline authorship are based on purported differences in style and theology. Since I am currently doing research in Colossians and Ephesians, I wanted to determine how significant these purported differences actually are.
One of the easiest and most objective ways to compare these letters is by examining their vocabulary. Using BibleWorks, I generated the following two charts. In the first, I compare the vocabulary of Colossians and Ephesians to the vocabulary of Galatians. In the second, I compare the vocabulary of Colossians and Ephesians to the vocabulary of Philippians. I also include in my analysis three epistles that are certainly not written by Paul (1 Peter, James, 1 Clement) and one pastoral (1 Timothy).
These charts demonstrate that the difference in vocabulary between Colossians/Ephesians and the seven undisputed Paulines is negligible.