Mistaken paternity would be readily apparent from a 23andMe test. There is a section where they compare family DNA, and a child is expected to share 50% of their chromosomal segments with their father. Of course, both child and parent would have to be tested.

I agree completely, the disease predispositions in the 23andMe service fall far from being of any value. http://Navigenics.com and http://www.deCODEme.com do a better job at this.