Chain length after new chainset?

Ok, so I just got a new chainset. New one is 48/36/26. Old one was 42/32/22, I believe. It's been a couple of days and it's just occurred to me that I might need to add a few links. I have the links and just need to grab a new pin or two. So as far as I understand it it's possible I need to add them [due to the larger chainset], I will check soon. Although I would appreciate knowing the exact way to check what's the right length, ideally without removing the chain, so I don't have to guess. There was also something else that I wasn't sure if it was an indicator the length was too short; I can't remember which chainring it was in but when I tried to shift into the largest sprocket it seemed to have trouble - skipping between the largest and second largest sprockets. I initially put this down to the rear derailleur needing indexing but now I'm not so sure.

Any thoughts?