Question 13.2 from the first paper of 2008 asked the candidates to list the features of a palpable left upper quadrant mass which might help you decide whether it is a kidney or a spleen. It is almost a shame to dedicate an entire chapter to the kidney-spleen controversy. In brief, the differences between spleens and kidneys are mobility, ballotability, and edge palpation.
Of course, if something is worth doing, its worth over-doing. In that spirit, Let us talk about the palpation of the spleen. The most detailed review of this issue is a 1991 article by Barkun et al, "The bedside assessment of splenic enlargement." A good overview is also offered in Chapter 150 ("Spleen") by James O. Armitage, from the 3rd edition of Clinical Methods: The History, Physical, and Laboratory Examinations.
Grover, Steven A., Alan N. Barkun, and David L. Sackett. "Does this patient have splenomegaly?." Jama 270.18 (1993): 2218-2221.
Barkun, Alan N., et al. "The bedside assessment of splenic enlargement." The American journal of medicine 91.5 (1991): 512-518.