Typically you will try to remove samples if and where feasible. Perform chemical analyses and tensile tests to see what modern grade the components may be similar to. In particular, you should pay attention to impurity levels (S, P) and calculate the carbon equivalent, which I think is in AWS D1.1.
[I once did this for a cast hydro turbine dating from 1913 and was surprised at how good the chemistry was.]
If you cant take a trepanned disc then remove grinding chippings from the steel and get analysis performed by leco and spectro analysis and test hardness to estimate UTS. As Brimstoner has indicated be careful with impurity levels in the steel. High impurity levels will be indicative of low toughness.
