Comparing different year group-sets I'm in the market for a new bike - don't worry I am not looking for a recommendation. What I am trying to find out is how do I know what groupset I am looking at? for example, I know Shimano 105 has been around for a while, but my understanding is that the 2017 105 is probably more advanced than a 2005 Ultegra. So, when looking at bikes that both purport to have Shimano 105 or Tiagra or Sora or whatever, how do I know what year the groupset is and how do I know whether the 2017/2018 of a lower end model is better than a few years old higher end groupset?

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.