I do not recomend pulling off battery cables to check charging systems. Most late models will die when you do so anyway and you run the risk of damaging electrical components. Remember these cars/trucks have several computers that are very sensitive to electrical changes. Most automotive shops will check for a minimal fee with the right equipment.
Most vehicles will give you signs of battery/alternator problems, ie: Slower to start, weak dash lights, etc. That's when it needs to be checked, not when it's dead.
Battery quality from some of the big chains may leave something to be desired. I worked for one at one point and the number of battery comebacks was amazing. Remember, they beat the manufacturers down in price to enhance the bottom line, guess what follows....
I work in the automotive industry and we always provide customers with electrical problems written printouts to justify the replacement of batteries or charging system componenets. Merlin, I would look for that from your dealer, and remeber batteries kill alternators and alternators kill batteries.....refer back to 2nd paragraph.