If the two birds are kindly sitting side by side it is easy to tell them apart. If you have a good feeding station with lots of suet then you do sometimes see them side by side. However, this is not always the case and people get confused. The problem is compounded when looking through binoculars as it is hard to get a size reference. Everything looks big though binoculars. Now-a-days I can tell in an instant which is which because I see them so often. Here are some tips for those who are still unsure and need to know what they are looking at. Look at the beak of the bird in the photo above. It is relatively long and has a stout strong look to it. Compare that beak to the one in the photo below.
(Photo Credits: all photos in this post were taken by me)