I am very sorry to disappoint you, but there no gold standard to assess frailty. There are tens of tools, and every researcher uses the one he feels the more suitable fore his practice. Each tool has its strenghts and its limits.
I agree there is not a golden standard tool for screening frailty. However, Fried Frailty index is the most recommended tool by researcher and many studies evaluate the validity of different frailty tools by comparing them against Fried tool.