Comparing different year group-sets
Shimano groupsets don't get upgraded every year. Each line gets major upgrades every four years or so. Have a look at the Shimano Wikipedia page, road groupsets section for a good overview of generations and when they became available.

You should be able to ask any bike store staff what generation of groupset is fitted to any bike they offer and get an honest and accurate answer.

If you want to check if a bike is fitted with the current generation, take pictures of the read derailleur, cranks/rings and shifters. Compare them to the pictures of the current generation on the Shimano web site. Each generation is fairly visually distinct.

Older generations can be identified by Google image search for specific component identifiers, e.g. Shimano RD-5700 for the previous gen 105 rear derailleur. The number of speeds in the cassette can help identify the generation too.