It's definately harder to find people to do decent chroming imho, around my neck anyways. The fact that these pieces can be done over 60+years ago and still have relatively decent finishes, is an achievement and testament to the operator in it's own right
How to tell chrome from nickle, as Ed said, chrome usually has a very dark, deep, shiny bright lustre, where as nickle, is notably much more subtle and faded out in comparison.
Not forgetting also, triple plated chrome is a mixture of nickle, copper, and chromium together, but i doubt that's used on firearms period because of tolerance/thickness issues.